I have to admit that I have never heard of an AV called Bullfighter, but one called BullGuard, so I will have to assume that whatever it was the shop has removed all traces, both on the program folder and registry, etc.
If you fully uninstalled avast and do a clean install, I wonder if the windows boot would be as normal or still experiencing this slowdown. If it does then you could contact the shop and see if they have any ideas. If not re-install
Before you uninstall it is best to have everything ready to reinstall so you don't go on-line unprotected.
Ensure you have your registry key that was emailed.
Download the avast! Uninstall Utility,
find it here and save it to your HDD so you can find it later.
Download the latest version of avast
http://www.avast.com/eng/programs.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Uninstall, reboot, run the uninstall utility, reboot, install, reboot.
I don't know if there are any Vista users on the forums that could help as many people experience of Vista is very limited.