Win98 won't reload if it detects that it is already there! When you did a fresh install a couple of months ago, did it go smoothly? Have you checked your system files subsequently to ensure they're not being corrupted (run SFC). I would be inclined to do a full format (even run Fdisk to be sure, if you want!) and reinstall. If the same problems develops, chuck your hard drive in the bin, buy a new one, and do it all over again!
Ok, let me get this straight because he is getting blue screens you would say to throw a hdd in the bin, hmmm, remind me never to let you touch my computer!!!
Ok PA28's advice of fdisking and reloading is gonna be the best a "start from scratch" install is what really is needed. If you still continue to have problems straight after or during the install, then you have some kind of hardware problem. Depending on how computer literate you are and how much money you want to spend you have several options.
1. Take it to a local computer shop get them to check it out and operational in no time.
2. Try locating what hardware is causing the problem, try swapping hardware, if you have any un needed pci cards for an install remove those, if you have more than one stick on memory try one at a time etc.
3. Bin the machine and buy a brand new one, from you local computer shop :D