Yours is an administrator account, or an ordinary account?
I will tell regarding Win XP, because I don't have much idea about Vista.
In XP, to check whats in a startup folder, you can go to:
C:\Documents and Settings\$Username\Start Menu\Programs\Startup
In place of $Username, name of your profile will be there. In the Startup folder will be any programs that are scheduled to run on startup.
There are other startup programs too, which do not show in the Startup folder, but still, they do run on startup. You can check such startup programs, with the help of softwares like
Codestuff Starter.
Another such simple software is
WhatInStartup from NirSoft.
You can use these softwares to know what programs are there in startup, and you can either look for suspicious programs, or you can also post the report here, and someone would possibly be able to tell something.
Since you are not able to login using your profile, then this should be done from the administrator profile.
Hope it helps.