Crashed Again - WHich AV to use?

OK - thanks to answers to previous email Vista up and running, dual boot with
XP.
Activated Vista - no problem.
So I wanted Antivirus protection on Vista and loaded Avast. All went well, I
registered. But email not set up on Vista - its on the XP system - so I
needed to switch back to XP to pick up the Avast code. Ran a virus scan first
- no problem.
Shut down - got an error message that flashed past so fast it was a blur.
Got the code from XP and reload Vista - heres where it starts
'Vista did not shut down correctly' - running CHKDSK. Hundreds off files
deleted. But Vista started up.
Avast did not start - used explorer to check folder - 'Folder name is
Invalid' can not change the name, can not uninstall Avast. Tried to use
Search - Explorer went unstable and fired off crash messages to Microsoft a
couple of times. Had to log off. re-booted later - 'This is not a genuine
version' in bottom right corner and Vista wont load - asked to load DVD/CD -
did so - nothing.
To cut a long story short - once again had to strip hard drive and recover
XP from backup.

DOnt know what went wrong - will try again - but anyone want to recommendan
AV for Vista?