Click Start
Type 'msconfig' into the box
Hit Enter.
(I think 'Enter' does the job with Vista but, if not, click on 'msconfig' in the entry above the search box)
Click the Startup tab
Remove the tick next to Realtek HD Audio Manager
Click Apply
Click OK
Restart your computer
When you see a warning box, alerting you to the fact that you're running with a disabled start up process, tick 'Do not show this message again'