Panda AV is OK-ish, but you'd be better off with either Kaspersky (includes firewall) or NOD32(AV only) - avoid Norton
Defragging your hard drive can make a bit of a difference - the one built in to Windows is OK but not great - something like
Diskeeper would be better.
Another thing is to clear out all the crap such as temp files etc on your system - try using the free utility
CCleaner (uncheck the install Yahoo Toolbar option when installing)