Avast 2015 on Windows XP SP3 has also the problem that it
slows SUMo down, and consumes much CPU (in my case around 50%, less than your 98%).
So maybe you have that problem because Avast has the same incompatibility with one of your background programs.
The culprit of the SUMo problem is "Scan programs when executing" of "File System Shield".
You can try disabling it temporarily to see if it is the culprit of your problem too.
If not, you can try disabling the shields one by one to narrow where the problem could be.