Can't update virus defiinitions

[ms.pc]

Hi:
I'm running Windows XP Home; just got rid of lots of viruses, updated windows xp to sp3, and downloaded the AVAST free 5.0 anti-virus program.  According to AVAST, the virus definitions are up-to-date.  But according to Windows, the virus definitions are not updated.  When I go to update them, the correct window opens, it says it is initializing, the narrow window signifying action begins to fill about 2% and then freezes.  I have tried this multiple times and the same thing happens each time.  Either a) windows is not recognizing the virus definitions or b) there continues to be a virus on the system.  The definitions are labeled version 101011.
Do you have any idea what the problem is?
Thanks for your help.

[Eddy]

Latest vps is 111209
Ok, now for the problem....
Do you or did you have other security (av) software installed?
If so which/what one(s)?
How exactly did you get rid of the malware?
What software did you use?
What have you done manually (if any)?
Did you ran a bootscan after installing avast?

[Tarq57]

Did you reboot after installing SP3?

What programs did you use to rid the computer of its viruses first? Is there another AV still on the computer?

Did you reboot after installing Avast?