If you are on a PC, check in your Windows\System directory for an .exe file titled winlgon.exe. If it is there, you must kill this bastard immediately. It won't be picked up by Norton or other antivirus progams that I am aware of.
winlgon.exe is a trojan that emails SPAM through a backdoor access it creates, so it doesn't use your email program, but can clog your access and slow things down. Most importantly, it's SPAM program. And having my computer hijacked for SPAM purposes is akin to being mugged or raped as far as I'm concerned.
If you see this .exe file, you must hit ctl+alt+delete, locate winlgon.exe in your processes and end the process.
Do not confuse with WINLOGON.EXE - that one is needed.
winlgon.exe = END PROCESS
winlogon.exe = LEAVE ALONE
Once the process has been ended, go into Windows\System and find winlgon.exe and DELETE THAT BASTARD, then make sure your recycle bin is empty as well.
It took me a week and a lot of money to get this taken care of. Ultimately though it worked out in the end, the whole thing was a rather nerve-wracking and expensive process. I hope you have better luck than I did.