avast is lighter on resources than many other AVs.
Currently my XP Task Manager shows a Mem Usage of 4,520KB and 0% CPU cycles when it isn't scanning, that will obviously go up when scanning is in progress as it is the main scanning process (a fact of life when it is working).
What is your Operating System ?
What is your systems CPU, RAM ?
Increasing RAM is relatively cheap and will improve your systems overall performance.