Don't keep exe or picture files on the desktop, this can slow the computer down.
Carry out regular defrags - once a week is what I schedule to keep the hard drive running smoothly.
Add more system memory (RAM) to the motherboard.
Upgrade the CPU (processor)
This one I mention now is probably one of the most useful for general use - get rid of start up programs that are not required! this will make the computer boot up faster and use less RAM once booted into the OS:
start > run > msconfig > press enter > start up tab > uncheck applications you are *sure* you don't need starting up with the computer - if you don't know what a certain program is google for file name and you will find out.
some people say to create a partition just for the page file itself (virtual memory) to reside and this is said to speed up performance a bit.
you can follow blackviper's guide to stop windows services that arean't required to run - but once again be sure you only disable ones you do not need else you will get problems.
the blackviper guide is very useful.
you could also create a RAM drive using a program such as RAM drive XP and run programs from it so programs will load quicker due to loading from RAM and not the slowest part of most computers - the hard disk - this is only applicable if you have enough RAM however and is not suitable for large programs but is suitable for smaller programs.
if you are into benchmarking with superpi you can set the priority to high and gain a few miliseconds.