Afterinstalling Avast on Vista 32 bit won't boot

???
On AMD Phenom processor with 2GB Hyper X Ram the hard drive light flashes for a fraction of a second green power light is brightly lit but Vista doesn’t boot. What could cause this?

The presence of another antivirus could do this… Do you have any? Norton preinstalled?
Can you boot in Safe Mode and uninstall avast?