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As soon as the PC restarts, immediately start tapping the F8 key continuously until the "Advanced Startup Options" screen appears.
Use the up/down arrow keys to select "Safe Mode" and press <Enter>, Windows should then start in Safe Mode after a long list of
file operations has finished flashing by. Safe Mode will look very odd compared to normal Windows so don't worry.
Now uninstall Viper Antivirus completely, then restart Windows. Hopefully Windows will run properly now.
As for Viper Antivirus, it seems not to be compatible with your system or with something that was already installed so you'll have to choose a different AV product. Presumably you didn't try the trial version before purchasing a licence?
Edited by phillipcorcoran, 04 March 2011 - 04:43 AM.