As you may already know, unlike an Antivirus and Firewall, it's possible to have more than one Antispyware program running in real-time
on your computer. Whether or not you will suffer any slowdown I can't say as everyones system is different, but I can tell you that SAS Pro runs very light, around 500k in Task Manager.
I myself have McAfee (which also contains Antispyware), Windows Defender and SAS Pro all running in real-time on Vista with no problems (that I know of anyway
Why don't you try the trial version of SuperAntiSpyware
first before you purchase a license and see how it goes.
Hope this helps.
Edited by RedDawn, 03 January 2009 - 12:04 PM.