If I could offer up 1 more small, but hopefully relevant piece of advise ? (no charge....unless you think it's worth something...lol
Ditch GUIMiner and get on the CGMiner
I use it on all my Win7 x64 machines and it rocks. It has the ability to use a config file, but I still find it easier to launch it from a BAT file.
1- Create a new TXT file in Windows.
2- Paste in the following (change where needed)
start /affinity 1 cgminer -o http://YOURPOOL.COM:8332 -u YOURUSERNAME -p YOURPASSWORD -I 9 --gpu-engine XXX --gpu-memclock 300 --auto-fan --temp-target 69
3- Close TXT File, SAVE, RENAME the file something
.BAT (have 'hide known file extensions' disabled, or it will remain a TXT file, regardless of extension
4- Right-Click the BAT file, choose EDIT (opens in Notepad), Make the required changes, adding your POOL, USERNAME, PASSWORD, GPU Clock Speed (replace XXX). CLOSE/SAVE
5- DOWNLOAD the latest CGMiner (2.1.2) for Win32 (no x64 version, doesn't matter, runs fine). Unzip it anywhere you like.Thread
......... Direct Download Link
(From Developer) - http://ck.kolivas.org/apps/cgminer/cgminer-2.1.2-win32.zip
6- CUT/PASTE the something
.BAT file into your CGMiner directory and Double-Click it.....VOILA!
The above CMD is set to run MEM @ 300Mhz and your Target Temp with Auto Fan control at 69C, but these values can be changed also.
Regardless of whatever driver, the CMD is also set to run on a single CPU core, rather than 100%, or all 4 cores of your Quad Core, if that's what you have (affinity = quick fix for 100% CPU Bug).
This will automatically control all 3 of your cards and give you 2 mining threads per card (default setting). Once you get your 'feet wet' with it, you can explore more options.
What I provided above is a basic (pretty much what I use exactly) setup for most of my cards, including my quad 6950 Miners (using I=9 Intensity as above).
Give it a try....you WON'T be dissapointed.
CGMiner replaces Overclocking Software, Fan Control and Miner - ALL IN ONE