So this happened after you installed Avast?
By delete you mean you don't have time to go to add remove and uninstall avast?
Isn't the shut-down-after-scan or something checked in your Avast? Maybe it does a quick scan from the startup option, and if the box is checked then shuts down automatically. Check that!
Or, it deleted a fake virus , something needed for the system.
Can you get into safe mode? (usually F8) If so, check the above, and you could try a system restore also.
Also, when it loads normaly ( you said it stays for about 5 minutes) then, go to start>Run> and type msconfig
On the General tab check Diagnostic startup and load only basic drivers. Click ok, it should restart.
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