My profile is over quota and it appears to be because of Thunderbird's profile. How can I fix that?

If you use Thunderbird extensively to read email and news, its profile may become so large that it causes your Windows profile to go over quota. If you have a desktop machine assigned to you, you can specify that Thunderbird store its information in a location other than your Windows profile (e.g., to the D: partition).

See http://www.mozilla.org/support/thunderbird/profile. The last section, Move an existing profile or restore a backed up profile, explains how to move your Thunderbird profile to someplace other than its default location in your Windows profile. If you do this it won't be taking up space in your roaming profile, but it will only be available when you're logged on to your desktop PC (i.e., it won't be available if you're on a PC in the lab).