Okay, first of all, you need to designate a hard drive that has plenty of free space on it as a "scratch disk." Photoshop NEEDS this, so you can "undo" things to multiple levels. You do this under preferences ...

Then, once you've done that, create a new document, and AS you create it, set the resolution to 72 dpi, if it is something you just want to see on screen. Also, doubleclick the magnifying glass so you are viewing your document at 100%. That also will help.