I don't fully understand the problem though, are you entirely not able to use TFF forums whilst logged in on IE. Or are you even unable to log in all together?
point taken.
however, sometimes, the solution is an alternative. i have come across situations where problems like this are not, truly, fixed until windows is, totally, restored. often a solution, to avoid this, is to use an alternative method, to achieve the same end.
though it is sometimes so, its unlikely its a virus. more likely the fact that OS's, over time get confused. particularly Windows, but it applies to all OS's. as items are installed, and uninstalled. the registery gets cluttered. at some point an OS will make a call that is identical to a call in a messed up section. this causes problems like that of the OP, and a great many others.