As mauserme said, it would be unwise trying to change the AV program in the middle of an infection, and we really need to know the name of the infection.
One thing you can try in safe mode is
DrWeb CureIT!. It doesn't need to be installed: just run the program.
Also try the usual free adware/spyware scanners, again in safe mode. (These will all run happily alongside an existing AV.)
AVG Anti-Spyware Free (Requires Win2k/XP)
Ad-Aware FreeSpybot Search & DestroySUPERAntiSpyware FreeThese may well do the job: then you can remove any existing AV and install avast!
If there is no AV on the computer, you can install avast! You will be prompted to do a boot time scan after installing avast! Allow this scan- a boot time scan occurs even before safe mode, so it's obviously better. If you find the virus is disabling the existing AV or an installation of avast!, try the DrWeb scan in safe mode.
Many infections today are 'hidden' infections: if you find the 'virus' persists, scan for and remove rootkits (hidden malware), then run all the above scans again:
Panda AntirootkitBlacklightAVG Anti-Rootkit