Essentially there is little difference between the avast free and Pro versions, certainly for web browsing. I don't see any such slow operation on my XP Pro SP3 system, see my signature and let us know your system spec CPU, RAM ?
Is this the first time you have installed avast, if so when ?
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
What other security based software do you have installed (firewall, anti-spyware) ?