You're welcome.
The avastSvc.exe is the main avast scanning engine and would spike as it controls all the resident scanners. It is set to a fairly hight priority to be able scan what has to be scanned and return control to the other applications.
Whilst this is happening if the hard disk light is constantly flickering that is showing that there could be a bottleneck at that point. But there isn't a whole lot you can do about that (short of getting a newer faster hard disk, but this is costly in time as you would have to transfer all your existing data and programs. It depends on what type IDE or SATA, format is used (FAT32 or NTFS, which is better) the rotation speed, 4500rpm, slow to 7,200rpm fast and the amount of cache/buffer the hard disk has, 8MB minimal 16MB and 32MB now becoming more common. This buffer/cache is fast memory that allows the hard disk to effectively catch up.
On XP, 1GB RAM should be enough, 2GB would be better for overall performance. This would mean more could be done in memory and less pagefile used (this is on the hard disk), so would contribute to any bottleneck.