LavaSoft ad-aware isn't all it's hyped up to be - they lie about the number of malware they detect (iirc ad-aware multiplies the number of database by 1.2), it's file-scanning algorithm is pretty slow and easy to defeat, etc.
IMHO a better choice is
Spybot Search & Destroy which is also free, and doesn't have a big-brother version that costs money.
I personally use NOD32 antivirus because it's a no-nonsense and pretty lean_and_mean program, which also has one of the best heuristic detection engines. Kaspersky is nice as well, but has become a bit bloated, and the way it uses NTFS streams and rootkit-like method for hiding those streams is a bit icky

(see sysinternals' rootkit revealer forums).
Big problem with NOD32: it doesn't cache file status, so every time you boot your system, files have to be re-scanned, which makes first-launch of apps pretty slow (AMD64x2 4400+ here, and it *is* noticable >_<). Also, I haven't found a way to disable the background/instant file scanning that persists through reboots.