Something doesn't sound right. Memtest boots off a CD or USB drive. It is not run from within windows. When run properly, windows doesn't even come into play. Memtest doesn't care what OS you use. If you are running the windows based version of memtest, then yes you will get this error. 32 bit programs run in windows can only use 2 gig of application memory. 64 bit programs can utilize all the ram.
Anyhow, if you aren't booting directly into memtest, then you're probably using the wrong version.http://www.memtest.org/