Instead of changing any processor affinity settings when rendering in D|S there is a better tweak. Open the task manager and do the same thing as when changing processor affinity but instead of “Set Affinity” use the “Set Priority” menu item and set it to “Below Normal”. It doesn’t remember the setting change for the next time you use D|S but you will be able to use your computer for other things quite easily. This will also have the advantage of causing less of a hit to rendering speed than blocking the use of processor cores, unless you run another processor intensive program like Photoshop or a video app while rendering in D|S.
The main bottleneck and cause of freezeups etc. is usually insufficient memory and the only cure for that is running 64bit Windows (or Mac OSX I guess) and having at least 4 gig of ram. Anything that can’t be held in ram while rendering is going on has to be read/written from your hard disk and will slow everything down considerably.