Defragment means the computer rearranges files on the hard drive so they are in continuous sections. When writing files the computer just uses the first available space, so it can write part of the file, then run out of space and need to move to a different section of the disk to continue. Generally defragmenting the hard drive just improves performance a bit, but some people have found that it clears up various weird glitches.
Right click on the hard drive in Windows Explorer, select Properties, switch to the Tools tab, and hit the defragment button (might be slightly different in XP).
Many programs that add themselves to the task try beside the clock do not actually need to be there, and can interfere with resource intensive programs, such as games. Unnecessary programs include email or IM clients, web browsers, wallpaper changers, MS Office quick search indexing 'feature' and multimedia players. The real-time scans of anti-virus programs can be particularly problematic. Usually right clicking on these icons will give you the option to exit, change the startup options or temporarily disable the program.
Lady_Rain describes how to use the startup manager in Spybot in
this topic, to help free up resources and reduce conflicts.
