Avast is currently my favorite anti-virus program. It will run significantly faster than Norton or McAfee (especially on Windows XP machines), and it catches more viruses.

AVG is also a top-notch anti-virus program. Computer Mamas is proud to recommend them.

When faced with a very slow computer, the first thing I will usually do is uninstall Norton or McAfee and install Avast or AVG. Sometimes this enough to effectively fix the computer. Norton has made great strides with their very newest product, but I'm not ready to let them all the way out of the dog house yet.

Malwarebytes is a must have. It's an excellent piece of software and will find things that your Anti-Virus program misses.