I think chances are your computer got infected by trojan horse and your gmail password and id has been hijacked. while it may be a big issue now you might want to consider even bigger issues, have you done internet banking using your computer recently?
Gary from name1price dot com.
i think you have to do a scan of yr harddisk
1) AVG is just a free AV software and it take up alot of resource and slow down yr system. It is not very powerful to capture the virus lar.
- Just do a scan with yr AVG see whether any virus detected??
2) Before you install a better one pls unstall AVG (normal procedure - pls take note only 1 antivirus is allow to install on yr pc !!!!!)
1) anti-malware
2) Norton Antivirus (need to pay or borrow from someone)
http://en.wikipedia.org/wiki/List_of_antivirus_software
3) McAfee
4) Others
try downloading Eset Nod32 antivirus trial and scan ur com for viruses, and then use malwarebytes antimalware as 2nd scan.