Recently, my friend's PC came to a grinding halt. It was quite messy and the system was full of cruft so I decided to do a clean XP Professional install. The system is a Turion64-based HP laptop (Pavillon dv5000) and is a few years old. The original symptoms were extreme sluggishness and constant swapping, which I assumed was probably due to some malware and a few (useless) AV programs running at the same time.
I did a clean install and as I expected, it all went away. It worked all righ for about a month and suddenly it all came back. No new software was installed between then and now, the system was protected by Avast, it was pretty much just used for Word. Now it takes forever to boot, most operations take a long time, for example copying a file from a flash drive to disk makes the CPU jump to 100% with the disk thrashing constantly. The same happens when playing some video from YT in Firefox. I tried investigating with Process Explorer but it turns out that in such situations, 100% of the CPU is consumed by "Hardware Interrupts and DPCs". I've no idea how to work with that. I'd be most grateful for any suggestions. I'll probably have to reinstall XP again but I have no guarantee this won't happen again so I'm quite demotivated as you might imagine.