i think there may bad sector in your hd,or it is infected by virus which slowen don your system.so follow the instructions
1.first of all defragment your hd,just right click on hd drive(like c d),and go to property>tool>chech now,defragment now,wait for some time it defragments
2.if not solved then just format by taking windows cd.
instruction to format
1.insert windows cd
2.restart,press any key to boot from cd
3.wait while files are being loaded
4.to set up windosws xp press enter
5.press f8 to agree
6.then press esc to setup new windows
7.delete all drives(sure you have taken back-up)
8.make new drives,just follow the instruction,it is easy.
9.then install windows in the drive by formatting
10.wait it automatically install windows,it will reboot ,but dont press any key when asking,just wait windows will setup