PC Locks up on boot

I have just downloaded and installed Avast Home. The installation went OK and I ran a scan. After rebooting, after logging into XP Pro, the task bar starts to build and then the PC locks up. Rolled back the PC and removed AVAST and everything is back to normal.
Any ideas please? Don’t want to reinstall and experience this again if it can be avoided.
:cry:

Did you remove your previous anti virus software?

Yes, I did remove my previous AV.

What AV was that and how did you remove it?
Do you have any similar tools installed?