At my house, we're Macintosh fanatics, so I don't know if the following advice will be relevant. Also, I haven't read all the posts in this thread so I don't know if the following suggestion has been covered. Hard disk drives have become as cheap as water (almost), even the external ones. If your computer has a USB 2.0 connector (the older USB 1.1 is way to slow for this to be considered) or firewire, you can buy an external USB 2.0 or firewire hard disk drive that you just plug into wall power and then connect to your computer with one cable. I've seen 120-Gigabyte external drives for $50. If PCs work like Macs, you can then load your operating system disk (I think this is Windows XP Home in your case) onto the new drive and make that your system drive. For any software that isn't working right, just reload or download a fresh copy onto this new drive. You can also copy files, folders and applications over to this new external drive. You can now also run a disk repair utility on your internal drive, which is a dangerous thing to do when that internal drive is your only drive and/or your system drive. This fix assumes the following:
1) you have an USB 2.0 or firewire port on your computer
2) you have the original Windows operating system disk that (hopefully) came with your computer or that you bought to upgrade your computer (I think you can buy Windows XP home for about $60 if you don't have it)
3) you have $50
4) the PC/Windows system allows the operations I describe above. (any other posters with PCs care to comment on this)
This is cheaper and less intimidating than the process of buying a whole new computer, but if your using a 900Mhz computer and can afford it, you should certainly think about entering the 21st century (at least as far as electronic technology is concerned) and buying a new unit. The money you spent on the external computer won't be wasted. It will work fine with the new computer and might be a good unit to use far monthly backups. (Having gone through all the aggravation you have suffered, once your problem is resolved, you will have learned your lesson about backups, and will certainly perform monthly backups for at least the next two, maybe even three, months.)
If even this approach seems too intimidating, your last resort is the SBE solution. SBE stands for Single's Bar Expert. Just go to your local single's bar. find some guy who has all the expertise you need, and get him top fix your computer. He will not expect anything in return for his help.