This is a Pentium 2 400 Mhz processor. These units shipped with Windows 98, so I feel sure that Windows 2000 was loaded later. Do you have a disc for this and a COA to make it legal?
In any event you should test the hard drive and RAM integrity.
Go to the appropraite drive maker's site for the hard drive, and
www.memtest86.com for the RAM test.
If this all tests OK, Windows will need to be reloaded.
I believe that model originally shipped with a 6.4 gig drive. If the size of this drive is different it could be important.