I have windows 7 ultimate 64 bit installed on my PC and i have been using Security Essentials for a while. it worked very good for me. but i noticed that some times when i connect my flash to my some of my friends notebooks which had the 32 bit version of Security Essentials installed, it catch some viruses on my flash. Is it possible that the 64 bit version of the software unable to catch 32 bit viruses.
It's definitely possible for a 64-bit Windows system to be infected by 32-bit viruses. So long as there is some sort of 32-bit compatibility a computer can be affected by a 32-bit virus.
It's pretty safe to assume that MSE will detect 32-bit viruses on a 64-bit OS because if they didn't then the Associated Press would've stabbed Microsoft in the neck by now.