yeah, McAfee is a resource hog, and not even such a great antivirus. I used to run AVG free, but recently switched over to Avast free, and it's excellent. If your McAffee version is the one that came bundled with the system, then I recommend you uninstall it and install Avast.
For speeding up the system, this is what I'd recommend. Note that your hardware should be fit and running fine, otherwise troubleshoot the hardware first, if you suspect bad RAM or a bad hard drive.
Assuming you have no hardware issues, do this:
(1) Thorough anti-virus scan with free editions of AVast or Antivir and the latest virus definitions file. But don't install or run more than one anti-virus at the same time.
(2) Anti-spyware scans with Spybot S&D and malwarebytes.
(3) All Microsoft security updates applied.
(4) Run disk cleanup to delete old temp files and other junk
(5a) Run chkdsk once to scan for file system errors. http://support.microsoft.com/kb/315265
(5b) Run a thorough disk defrag. Use a good defragmenter for this, and also run a boot-time defrag to fix the MFT and page file if they are fragmented. You can use Diskeeper 2009, which is excellent. Their website has free 30-day trial versions. http://www.diskeeper.com
(6) Disable unnecessary background processes and services that take up memory. You can kill all the useless third-party programs/updaters like Adobe, google, Quicktime etc that have placed themselves on the startup list.
(7) Make sure all the hardware components have the latest and most stable drivers.