I've never tried mods, because of this article:
http://www.minecraftwiki.net/wiki/META-INF"This directory is notable because it must be deleted to use mods. The META-INF folder contains a digital certificate (MOJANG_C.SF and MOJANG_C.DSA) that contains a checksum of all the Java files. The certificate is issued to prove the code is safe, and is what lets the web applet version of Minecraft run without Java safety dialogs. When the files are modified, they could have been tampered with, and the certificate is invalid (the checksum changes). If a .jar file contains an invalid certificate, it won't run. Removing META_INF removes the certificate, so it will run again, but if you try to use the browser version of Minecraft, you might get warning dialogs. The MANIFEST.MF file is unrelated, and simply contains information about the archive."
That only applies to the browser version, and I can't fathom why you'd use it anyways.
-Canama
I don't have administrator access to my laptop (only to my Windows 7 computer, which can't go online). :-/ If you have a workaround, I'm all ears!
