AVG Free Advisor - AVG Anti-Virus Free Edition
AVG Free is listed as compatible with XP 64 and Vista 64
AntiVir PersonalEdition Classic - More than Security
Avita Antivir (Free/Personal/classic) is also listed as compatible
avast! Antivirus and Windows x64 Edition
Avast also supports X64
Are you sure CA don't?
Just looked, the "2008 version" does not support Windows 98/ME, XP 64 or Vista 64
Dropping Win9x support is understandable, but failing to take on board X64 issuses, when every new processor will be X64 capable and the rise in memory demamds may make X64 essential is incomprehensible.
So, there you have it, the big 3 of free ones all support it, so do you choose:
1. AVG - low false positives, but often reckoned to be weak if stretched further than the common wild list
2. Antivir - plagued by false postives and hairtrigger heuristics
3. Avast - must re-register yearly for a free licence
Reading up on the changes in Avast, it seems pretty comprehensive, though it can lag AVG in tests.
AV-Comparatives
Avira does very well in detection, very well in "proactive" detection (finding newer viruses by hueristics without a signature update. It keeps getting marked down for false positives, and would be a almost solid top of the table were it not for that issue.
As for AVG, I've seen it miss some, it seems that it's slow to get signatures for anything other than the major threats, and it was transferring files to a new system, when CA spotted them - it may also be the case that CA defaults to deeper inspection of zip files and stuff, as most were inside unopened compressed files, including a "live virus" sample file I'd completely forgotten.