Heres a few things you can try
Start>Run and type "msconfig".
On the "General" tab click "Normal Startup- load all device drivers and services" and click OK.
Run HjackThis and post the log
After that use this (permantly deletes all temporary files)
When you use this you need to be in safe mode. to go in safe mode press F8 on the Windows xp boot screen. if you cant run in safe mode just run the program normally.
If its not a virus problem, it is probably java. uninstall the add on. Then go to Program files/Common files and delete the java folder. then reinstall java.