Maybe your computer has not enough power to run a 64 bit version? Give your specs and build please.
I would suggest that ANY CPU these days that is 64 bit capable is MORE than sufficient to run W7 X-64 (any build). Even on a 512MB RAM machine (1GB will be more than OK - even for W7 X-64).
If the computer is slowing down hideously do the following -- switch off any AV software first as these can be quite a resource hogger. I've assumed that you've already checked the computer hasn't been infected in any way.
Check also if your "Disk Led" is on a lot or flashing vigorously -- loads of disk activity will kill any system.
then do :
1) Unplug the Internet to stop any spurious activity such as Media streaming (a lot of people stream particularly audio and never turn it off).
2) Check you are not running out of Disk space. This will cause problems particularly if the system has to do a lot of paging.
3) Stop unnecessary system services from running -- I'm not an expert on this - especially as a lot depends on what applications you have on your computer - but there's plenty of help on the forums.
5) kill indexing as well - especially if you have a lot of disk drives.
If these still don't work freshly install the OS again. W7 runs fine so you must have some activity which is killing your system. If the Disk space isn't a problem then my Nr 1 culprit would be the AV software you are using, followed by Disk indexing.
Cheers
jimbo