CPU usage hits the ceiling several times

Hi

CPU usage hits the ceiling several times during manual AV search.

I also experience slow response or no response sometimes. This may happen when running some applications at the same time. Even ctrl-alt-delete can stop functioning. (mouse pointer moves)

Have Pentium 4 2.8 GHz 512MB 90GB free
WinXPhome sp2 and bugfixes AV updated

Thanks in advance for every suggestion

\Gunnar