Is Windows XP or my Hard Drive dying?

Hey everyone. Been a while. Just recently, my whole PC has slowed waaaaaaaaaaaaaaaaaaaaaaaay down. Here are a list of sypmtoms:

1. I click on an Icon and 30 seconds later it opens.
2. I type letters and they don't appear for several seconds.
3. Software appears to be locking up.
etc. etc.

Mind you that this problem came on rather suddenly. Everything was working fine and then boom! I went from 150 miles and hour to 20 miles an hour. I will tell you my one suspicion: I had Win98 installed on this PC (FAT32) and then I installed WinXP Home Edition the Upgrade version over the Win98. I didn't want to lose my files or have to slick my hard drive, so when it ask the question: "Do you want to use NTFS?", I told it no, because I thought it might erase my files.

But, it worked great for a while, but now seems to be biting the dust. I've run reg cleaners, rambooster, adaware, hijack this, anti-virus and defragged to see if this may have helped - and it did some, but something is still making things slow.

Bottom line, could my HD be dying or is this more than likely a software issue?

HD is 5 yrs. old
Rest is good stuff: AMD 754 2ghz, MSI mobo, ATI Radeon 9800 Pro 128MB vid card, Lite-on cd-rw, Corsair Ram, 500 watt PSU etc.

Any help on this problem is greatly appreciated,
Thanks so much, StratMan23