computer got slow

Since I’ve installed avast! 4 home on my computer it’s pretty slow.
win xp sp1; 2,4 ghz; 500 mb ddram
Does anyone have an idea?
Is there an option like realtime scanning?

Please read here: http://forum.avast.com/index.php?board=2;action=display;threadid=5140

Which version of avast do you have? (See about dialog right clicking the system tray)?
Do you set avast! at High sensitivity? Do you have too many and big archive files (such .zip or .rar)?