Guys you are talking KBs here when in fact it is MBs e.g. 18,352K is 18,352,000 or roughly 18MB.
The other aspect is that this avastSvc.exe is the control for all shields so it is likely to spike/fluctuate depending on what you are doing at the time. Not to mention RAM use is not related to CPU activity which the OP is asking about.
@ Morpheus666
When the CPU spikes to 100% is the avast icon actually rotating, indicating that it is scanned ?
What other security software do you have installed, firewall, anti-spyware, etc. ?
By a fresh install of XP SP3 on an old Dell, was this the dell recovery disk that came with the old dell (as they often come pre-installed all sorts of junk including another AV) ?