Hi vale225, welcome to the forums.
For a start, what avast providers are you using (standard shield, Internet Mail, IM, P2P, etc.)? The more you use the heavier the load.
What are the settings of your standard shield on-access scanner? The more sensitive the level, again the heavier the load.
What other programs are you running? What does your Task Manager show for processor load and what is using it?
Have you noticed this since installing SP2? There have been instances of the SP2 update slowing all sorts of things, the internet being just one.
Without this information, we can't really hazard a guess.
Example:
I barely notice any slowing on my system which by all accounts is much less poweful than yours, see my sig.
P.S. Please switch of the CAPS LOCK.