The clues are too slim here. Try the old test we use at the shop. Slip in a blank HDD, install a quick install of XP and see if the problem remains. If it does, it's a hardware issue, if not it's the OS/driver or hard drive issue. Cloning the HDD to a new HDD is the test to see if it was the HDD.
Repairing the OS is not done as much as it used to. Why? Folk have lost patience and don't want to deal with or learn this area.
Running windows xp home,asus board,amd quad core 920. Xp logo shows loads very slow,then black screen for at least minute or more. Then login screen, and still runs slow.defraged,ran chk disc. Ran malware, kespersky virus removal tool found exploit,and Trojans removed.still slow.help!!