I'm running a dual boot system with XP as my primary and Vista as my secondary OS. Both OS's are on the same drive but on seperate partitions. So my pc was working fine yesterday. I boot into XP today after an abnormally long boot time of about 4 minutes, to find that one core of my cpu is running at 100% most of the time. The other core is fine. I ran process explorer and it turns out the problem is with the Hardware Interrupt which is constantly using 25-50% (1 entire core out of 2). There is also constant hard drive thrashing which appears to be the cause of the abnormal cpu usage. I assumed it was a hard drive failure so I rebooted into Vista and the problem is gone. Working perfectly. The XP partition reads and writes at normal speed as i tested with some large files and there are no problems. I reboot into XP and again a long boot time with the same cpu usage problem. I've updated anti-virus, as-aware, spybot, windows defender etc and my system is clean. So i'm now stumped as to what is causing the issue. I could easily reformat, but i'd rather not. Any ideas?