why does it shows something like backdoor?

when i used to use vista home 64bit it never showed that but when i switch to vista ultimate 32bit it shows it sometimes

It might be a good idea to do a scan with some of the programs in this guide:
What You Need To Know About Removing Infections and Securing Your Computer

If your computer repeatedly detects files it means that either you are infected or your AV if finding a false positive.