"Kernel_Date_Inpage_Error" and other problems.

Here's what I know:

The Dell Inspiron 1721 has:
Vista Home Premium SP1
250 GB HDD with 170 GB free and 3 GB RAM
Athalon Dual Core 1.9 GHz processor.
AVG paid subscription with current updates - runs daily
SuperAntiSpyware paid subscription with current updates - runs daily
SpywareBlaster regularly updated
Windows Defender

Issues:

1. It started intermittently hanging up for several seconds when playing music that's stored on the HDD and when watching streaming videos from ABC and other networks.

2. Kept getting this message: Dell Wireless WLAN Card Wireless Network Controller stopped working and was closed. A problem caused the application to stop working correctly. Windows will notify you if a solution is available. The internet connection would stop working for a few seconds, then resume. Then stopped working altogether until today.

3. SuperAntiSpyware found two incidences of the trojan "Trojan.Dropper/Gen" - - which it reported that it had fixed.

4. Then, while listening to music a couple of weeks ago, the music hung up and starting skipping - - sounding like a CD does when it hangs up in a CD player - - then, the computer started beeping LOUDLY and consistently and then crashed to the Blue Screen "Kernel_Date_Inpage_Error." There, the computer hung up and sat there still beeping loudly with the music still skipping. After several minutes of listening to it scream, I turned it off manually.

5. After reboot, things seemed slower than its usual lightning speed. Also, Firefox started having intermittent episodes of 'shuddering' and sometimes reduces to a small window and then jumps back to normal. That's weird and NEVER happened before the crash.

I had recently installed iTunes and QuickTime before the crash.

How I wish I would have thought to just run System Restore when things started acting strangely.

But frantically trying to fix it, I did the following things, but probably not all in the order listed. And, there were probably other things I tried, too, but I can't remember them all (in hindsight, should have written it all down as it was done):

Ran (not particularly in this order) CCleaner; AVG, SuperAntiSpyware, Ad-Aware, and Windows Defender and they found nothing else. Downloaded MBAM and ran it and it found nothing.

Also:
Ran Chkdsk - it found errors and said it fixed them.
Ran PCPitstop and followed most of its recommendations.

And:
Uninstalled EVERYTHING I thought I could do without including (in no particular order):
Jasc Paint Shop Pro
DivX
TVTonic
Ad-Aware Anniversary Paid Edition
Google Desktop
Google Chrome (really liked it but problems started right after installing it
Adobe Air
Live Cam Avatar
Skype
Apple Mobile Device Service
Bonjour
iTunes
A couple of photo editing programs

And started Googling the problems, and did the following (not particularly in this order):
Turned off bluetooth.
Tried defragging, but couldn't get Vista to defrag, so downloaded Piriform's Defraggler and ran it. (PCPitstop had found that the computer was 43% fragmented.) It took 14 hours to complete and was still 34% fragmented.
Then, noted the files that wouldn't defrag - - several System Volume Information files and a file named i2api_debug.log that was over 1 GB big and growing.

So, I Googled some more to try to find out what to do about that, and then (not particularly in this order):
Deleted the i2api_debug.log file (which supposedly has something to do with a motherboard controller?), rebooted, and immediately set the new file the computer created on bootup to read only (that's supposed to stop it from growing again).
Defragged again. This time, it only took an hour or so.
Set a new restore point and deleted the others.
Unscheduled System Restore's daily and bootup auto creation points so the only one would be mine.
Downloaded (from Dell.com) and installed some new drivers' updates PCPitstop said were needed and Dell recommended for my service tag - - 1 driver for the WLAN Network Controller Adapter Card and 2 for Broadcom.

And:
Because the problem seemed to start with audio, Googled that and then:
Downloaded something called 'Codec Sniper.' It found:
All MS Movie Maker and Cyberlink codecs are broken.
Don't know what that means, is that bad? Also, don't know how to repair the 'broken' ones. (Not even sure what a codec is/does but Google results indicate it has something to do with audio/video play.)

Uninstalled and reinstalled Mozilla Firefox.

Firefox is still acting weird since the crash, 'shuddering' and flashing to a small window and back to normal - - even though it's been reinstalled.
After defragging, the hard drive was still 7% fragged, so defragged it again - - 3 or 4 more times.
Within minutes after defragmenting, the machine is 4-7 percent fragged again.
System Volume Information files are approximately 30 GB big - I think that's System Restore, right? Does it need that much space? That's why I stopped it from making any more restore points. It just has the one I set; however, if the machine isn't fixed, the one I set is no good, right? And the file won't defrag and is still huge.
Reran PitStop to see if the report was better, but still says it needs drivers - - the very same drivers I just downloaded and installed.

I need to add that a few days ago the computer mouse cursor began refusing to respond at times or refusing to go where I wanted it and just jumped around or zoomed to the top of the screen and jiggled around or disappeared completely. Bizarre!

Today, things go from bad to worse. I've been unable to access the internet the past few days, then today it just began working again; however, today - - another blue screen after the audio hung up again while playing a CD. Same "Kernel_Date_Inpage_Error." Since my job is Music Director at a church, I REALLY need the audio to work without causing all these BSOD's.

Hope that wasn't just too much info to wade through.
I'm not very 'tech' - - but can follow clear instructions pretty well.
Do you think the computer is infected with something I can't find?

Thanks in advance for any advice or help you can give!