Microsoft Security Essentials is adequate, and I usually recommend it to everyone. It is very lightweight, and is unobtrusive. A note though before anyone tells me it's bad: there are a few anti-virus software rankings and reviews online, and Microsft Security Essentials usually fares poorly compared to other AVs in these. However, the reason why MSE fares so poorly is because it lacks heuristics scanning; it only has signature scanning. However, this means the MSE has significantly less false positives compared to other AVs, and false positives get very annoying indeed.
Heuristics scanning is the analysis of a particular program's code for anything deemed suspicous. Signature-based scanning is different. Rather, the signature-based methodology involves downloading and referencing to a large database of suspicous files when scanning. If a file appears in this list, then it is marked as dangerous or suspicous.
A side note: signature-based scanning doesn't offer protection against zero-day threats, which are recently-discovered threats or vulnerabilities in software that haven't been patched yet. But zero-day threats are easy to avoid with common sense, and with common sense and vigilance you shouldn't need an overly-complicated AV anyway.