why not? i've been running a Windows 7 Ultimate via bootcamp in my 15" MBP since march and i'm a happy camper

It won't make your mac to run slower or hotter;as long as the needed drivers are installed prior to fully using Windows.
As for the viruses, it is possible to have virus on your Windows partition if you aren't careful, but it won't affect the OS X system as the virus is only written and can be read by Window applications/files.
If i'm not mistaken, you can permanently boot in Windows upon startup. You just have to tweak some settings via bootcamp assistant which I haven't tried yet.
Best bet would be to install Parallels or VMWare fusion in your OS X. This will enable you to use Windows without leaving OS X system (though mac will only allow you 1gb of RAM for Windows use)
OT: my SIL have been using her 13" mbp ever since with a Windows XP
