Have internet connection but can only log on in safemode
 

I am very computer illiterate. A few weeks ago my internet explorer could not detect the internet. I tried Mozilla Firefox and it worked intermittently but would never allow me to login to secure sites. I used the help and support index on my computer and it said to ping the IP address, which failed all 4 sends. I contacted Comcast, they were not able to help. They told me to disable all firewalls, which I did and it did not remedy the problem. I am only able to login via putting the computer in safe mode with networking and I login as the administrator. So for weeks I've been in safe mode. Is it a virus, spyware or do I need to look somewhere else. I have AVG free version and Ad-aware by lavasoft. I have windows xp sp2 and internet explorer v7 installed. Please advise. Thank you.