possibly harddrive problem. how old is the harddrive?
you can download drive fitness test Support - Downloads and Utilities
you can get the version that burns to a cd so you can boot off it , or I believe they have one you can put on a floppy as well.
boot off that. it will check your harddrive. pick the advanced test.
that will tell you if the drive is bad or not. anything red means you need a new drive.
one other thing. check your cables as well to make sure that nothing came loose. maybe swap an ide cable if you have a spare
you can also try to install windows on top of itself. as long as you dont format you wont loose data. but you mgiht have to reinstall some applications.
if windows fails install then that is another way to tell if the drive might be bad.
post back with any questions or any further info after either running the test or trying to reinstall