How can I tune my desktop for best performance?

what kind of operating system are you using? Windows XP, 98, 2000, ME
..or Unix etc..

Normally each operating system has it's own system tools, such as
defragment, scan disk etc that you can use to help "tune up" your PC.

You also may want to look in cleaning up som eof your temp files,
internet cache, cookies and history...you can do all of these things
your self, or find some free download tools from something like
download.com   Better to use the operating systems tools themselves
than the freeware most of the time.

For best performance, reinstall your OS.

Back up all your personal and important data.  Make a list of programs
you have installed.  Common stuff like Acrobat Reader, word processor,
spreadsheet, internet browser, etc.  Oh, and you will also want to get
all your drivers in order (not typically necessary for XP).  Format
hard drive.  Reinstall OS.  Enjoy the best performance you can get.

Alternatively, you can 

-delete files in temp folders
-uninstall unwanted programs (uninstall...not just delete)
-remove things from startup (for windows OSes delete things from your
startup folder in the start menu, run msconfig [for 98 and XP] and
remove unwanted programs in the startup tab)
-run adaware, spybot, and hijackthis to clean out spyware and addons
(careful cuz these can make your system unstable/unusable)
-defrag hard drive

Did you mean this? right click on my computer goto properties. click
on the advanced tab under performance click settings?...that help?

If you told us your system specs, it would help.  The easyiest and
fastest and cheapest way is to upgrade your ram, at least to 512 MB. 
You have to use the same kind (i. e. PC3200, sdram...) for it too
work.