Another +1 to Microsoft Security Essentials and 7zip.
MSE works wonders. Small foot print/low on resources, integrates well with Windows, and its protection is great. Don't let people tell you otherwise. You really don't need anything else unless you feel the need to pay for something lol
7zip is a great OSS file archiver. Its fast, lightweight, and will uncompress/compress pretty much everything.
ffdshow tryouts is a great codec pack that pretty much plays any type of media format.
Media Player Classic. An excellent media player that's great to play movies with an extremely small footprint.
Foobar2000. A great music player that's really low on resources and yet has a ton of features and plugins.
Microsoft Office. Simply put, the best office program out there. OpenOffice/LibreOffice is great and all, but if you do anything serious with Documents and whatnot, nothing beats it. Plus if you're a student, you can get it really cheap.