Antivir ==> Avast! ==> AVG
Unsure about CLAM, as it is an open source project involving the creation of a Windows AV application based on an open-source Linux app.
Antivir has low overhead and a good detection rate. No auto-update (work around using task scheduler), no e-mail plugin (but has on-access scan, so??).
Avast has a higher overhead, a good detection rate, and a shed load of features (auto update - engine and definitions, email scan, web scan (invisible proxy/ conflicts with zone alarm), P2P shield, IM shield, etc..). It is also 64 bit ready, if that matters to you.
AVG is ok. Low overhead. E-mail plugin. Auto updates. It's detection rate is not as good.
If you are freaking out about viruses, and are willing to pay, go with:
Kapersky (the best, hands down)
-or-
F-Secure (excellent)
Norton and McAfee are also very good (they had better be for the price!!)