to change what Windows is doing: go to Start> Control Panel>System... Select the "Advanced" tab, then select the "Settings" button in the "Performance" area...  select the "Advanced" tab... you have three very important settings here... My settings are usually... processor scheduling = Programs... Memory usage = Programs, and the one you're looking for "Virtual Memory"... Click the "Change" button. 
  You have three options for each partition listed, and they must be set separately... your choices are: Custom size, System managed size, and No Paging file...... I STRONGLY warn you that "No Page File" is a very bad idea I have three partitions...(This is for an example),  If you click "Custom Size" you will be asked to specify the size in Megabytes... I am set at 2000 for initial size, and 4000 for maximum size.... do not enter it as 2,000 or 4,000... next you click the "Set" Button..... If you have more than one partition, then I'd recommend setting a hard and fast pagefile for each..... Windows will, at some point, "suggest" that you let Windows manage your pagefile system.... working with graphics.... that's a bad idea. Hope this helps. Lou.
  Note This will not directly change PS's cache sizes, but it will prevent Windows from "moving in" on Photoshop's space. Lou
 

"..... and that was when things got interestiing."