I got some virus called av_dm.exe. Check your startup programs with msconfig to find where it is located, delete it from everywhere and untick these entries from msconfig startup.
It fixed the process which ate the CPU, but AVG still complained about a trojan in atapi.sys
I didn't know what to do with it, because AVG couldn't remove it, so I downloaded the service pack for XP manually (search for it on Google), and renamed it to .cab, so I can look into it. Renamed the trojan atapi.sys to .bak in case the fix doesn't work, copied atapi.sys from the service pack and rebooted.
This eliminated the warnings by AVG. Let me know if it works for you or if you have a better idea how to deal with this pest.