If you look at the requirements of 64 bit windows 7 it says 2 gig and 32 bit windows 7 needs 1 gig. This in itself shows you 64 bit uses more ram than 32 bit just to run and seeing as you only have 2.5 gig you could have as little as half a gig left to run apps specially after an AV is installed
http://windows.microsoft.com/en-GB/w...m-requirements
The correct advice in your instance is to run 32 bit