Can you elaborate why the performance goes down?
Also the main reason for all of this is my older brother doens't have his own comp and will be using mine for limewire, I fought with him now too, but he's 18 i'm 14 who do you think won?
I have a Vista 64-bit machine as well as my Macs. When I tot up how many CPU cycles it spends updating virus signatures (which are always out of date of course, scanners can only check for yesterdays viruses), defender definitions, then running the actual virus and malware scans, et cetera, and then tot up how many CPU cycles I actually get to use, I start to wonder if it really is MY machine.
It is a joy to just turn on a Mac and use it myself, just me, doing my stuff! I'm so glad I switched a few years back.
My experience of virus scanners is that they range from an inconvenience to some that are downright intrusive. Safe practice is always preferable I think.
As for your situation:
Make sure that you set up separate accounts for you and your brother, he shouldn't object to that. If you can get away with it, make sure his is just a standard user, that may be a stretch if he's savvy though.
Limewire? Ouch. There goes safe practice. See if you can persuade him to run Limewire in a Virtual Machine. When it gets infected you can just restore a backup copy of the VM.
Do you have an external for Time Machine? If so, partition it, use Disk Utility to create a bootable clone of your system, and use a separate partition for Time Machine. You then have lots of restore options should something go wrong. You can immediately boot into the clone, and restore differences from TM, then wipe the infected drive and copy the new version back. Or you could just do a TM re-install, but an immediately bootable backup partition is nice.
One of the great advantages of Mac over PC is this potential speed of recovery, especially if you're well prepared.