It is possible - you would need to create 3 standalone Windows instances and its usual to do this in 3 different drives or partitions.
The Vista boot manager will handle multiple installs if its the last installation (after XP or Win2k) otherwise a 3rd party tool like Vistabootpro is needed - there are obviously other ways round the problem but I wouldn't go there if you're worried about losing data.
The problem with your data is that all NT based versions of Windows store data under user ID's on their own partitions - the best thing to do in my experience is move all user data to a neutral drive or partition - easy enough for 'My Documents' but a little more tricky for mail and this needs to be done for each OS.
As each OS has its own registry all programs which rely on this would need to be installed on each which in itself can start to get tricky - especially if you get updates.
Also remember that (as far as I know) Microsoft only licence one instance of Vista with each pack (32 or 64 bit) even though you may have the software for both so will identify one of your Vista instances as illegitimate unless you've bought it twice.
No - it won't slow the system down but could give you a lot of grief !