Computer isnt able to find files after initial startup

[hot_java]

I have a compaq SR2050NX with windows XP version 2002 service pack 2.
About two weeks ago I came home to find I could not log on. It no longer recognized my passwords. I removed the motherboard battery and installed it the next day. I was able to log in and everything starts up but after about 5 minutes I can only move the mouse and access certain screens. when I look at my computer drive it shows that it is searching for the files but it won't find any.
I can shut down and start up and everything is fine again. I can get into the files but after 5 minutes it can no longer open new windows. I can move from panel to panel with what I have open and can even open some new things like the clock but once again it shows the hard drive empty.
I put this drive into an enclosure and it works fine with the laptop.
How can I fix this?

[Broni]

Go Start>Run ("Start Search" in Vista), type in:
sfc /scannow
Click OK (hit Enter in Vista).
Have Windows CD/DVD handy.
If System File Checker (sfc) will find any errors, it may ask you for the CD/DVD.
If sfc won't find any errors in Windows XP, it'll simply quit, without any message.