Desk top items won't do much to slow you down except during the initial boot up , but aps running will. Look in your system tray (lower right corner) and see how many apps are running. You can turn them off by going in to msconfig.
Another cause of slowness: not enough hard drive space. Windows uses some of the hard drive as virtual memory. In this case, no space means - no memory - means slower computer.
As pointed out, malware, spyware and viruses will also affect computer speed.
FWIW: Desktop icons are nothing more than shortcuts to programs. Your computer won't run any slower with a hundred of them than it would with only ten. Besides, if a cluttered desktop is a sign of a cluttered mind, what is an empty desktop?