Computer is making a bad noise that comes and goes

Hi my computer is making this noise kinda like a what a lawn mower sounds like in the disrance. I know that sounds stupid, but thats what it reminds me of. I tried researching the net to find out what it may be. CPU fan? Do I need to take it apart and clean out the dust? The noise is constant for the most part only occassionally quieting down.
I know this is really dumb of me but I haven't ever backed up my computer totally. I bought a 128 mb Lexar Jump Drive and have been trying to transfer all my pictures to my son's computer, but that is taking forever. I have about 2000 pictures on here. I have almost all my pictures transfered but I also have like 1200 to 1400 mp3's on here also. **Note right now while I am typing this the noise has quieted down to a very low hum** It's going to take forever to transfer that many mp3's this route and I am scared my computer's cpu will fry before I can complete the process. I have a cd burner on my computer with Adaptec Esy Cd Creator.
I thought maybe zipping them to compress them but will that cause me to loose the quality of the mp3's or my pictures? I am not even sure that you can zip those kind of files.
Okay the backing up part.. I am so lost! Where do I begin what exactly do I back up, how do I backup, do I open the folders and select all the items inside or highlight the main folder and save it somehow that way. I know this is so stupid. I have wanted to wipe my hard drive clean for a while now but there are some programs I don't have the installation cd's for and would like to keep them.I had my computer hijacked by this hotoffers thing last week and got it off I hope but I am sure there is other spyware or whatever on here still. I don't want to accidently save a file and it contain something like that in it. I would like to keep my favorites folder and some emails I have saved in my drafts folder and especially all my mp3's and photos. I am providing below my system info. I hope this is what you needed. Looking at this info it looks like my memory is really bad, but I am a bad one to have many windows open at a time while doing something else.
OS Name Microsoft Windows
Version 4.90.3000 Build 3000
OS Manufacturer Microsoft Corporation
System Name MY COMPUTER
System Manufacturer 00101900 XE813
System Model 15298400061042
System Type X86-based PC
Processor Intel(r) Celeron(tm) processor GenuineIntel ~768 Mhz
BIOS Version EPP revision 9.00
Windows Directory C:\WINDOWS
System Directory C:\WINDOWS\SYSTEM
Boot Device \\Device\Harddisk0
Locale United States
Time Zone Central Daylight Time
Total Physical Memory 126.45 MB
Available Physical Memory 228.00 KB
Total Virtual Memory 2.00 GB
Available Virtual Memory 1.70 GB
Page File Space 1.88 GB
Page File C:\WINDOWS\WIN386.SWP
I won't post it on this message but may I post the other things my system info says. Like conflicts and sharing, IRQ's, memory..etc..? Because there are so many items that are the same listed multiple times. I have no idea how you go about fixing them. I will wait for a response before posting that in case you rather me not.
If anyone can help please try to explain it like you was explaining to a kid because I am so slow when it comes to this stuff. I can't afford to take it to a computer specialist. Sorry this is so long, but I wanted to be fairly clear on what I was talking about. I hope I did it correctly.

 

So i am so hot gee thank you....

Smallville fan !!!

Yep it's your CPU fan or your power supply fan for the cpu just buy
a new one there pretty cheap around $20 and if it's your power supply fan
your could try to clean with a canne of spray air it's around $10.

ANd for a first post alot of info.....

 

yes, compressed air is pretty cheap, i picked up 2 cans at circuit city for $15. just turn off the computer, take the side panel of the case off, then spray out the dust.