It's fixed!!!! It all boiled down to the fact that I needed to update my BIOS. I've always been afraid to update my BIOS for fear that I'd ruin my motherboard, but I tried it out and it worked, no more freezing!
Thank you everyone for your suggestions and help, it means a lot to me that I was able to get my Windows 7 up and running. Special thanks to Nibiru2012, you were the one who was able to pinpoint the culprit. Windows 7 Forums rules!
For any ASUS P5Q Pro (or similar) motherboard users out there who are experiencing constant freezing in Windows 7, you'll likely need to reflash your BIOS. It was a fairly easy process, following instructions on one of ASUS's forums:
http://vip.asus.com/forum/view.aspx?...age=1&count=70
Preparation:
1. Document all your preferred BIOS settings (i.e. the settings you have changed from the default setting).
2. Download the BIOS file, which normally is compressed (zipped).
3. Unzip the BIOS file to a FAT/FAT32 formatted USB Flash Disk with only one partition (or to a Floppy Disk or a CD-ROM). If the unzipped BIOS file is named like e.g. 'P5B-ASUS-Deluxe-1101.ROM', you can rename it to 'P5BD1101.ROM'. Otherwise a truncated name will appear as 'P5B-AS~1.ROM' due to a limitation to max 8 characters in the filename (8.3).
Update:
1. Reboot.
2. Press (tap) <DEL> during POST (Power-On Self Test) to enter BIOS Setup.
3. Load BIOS Defaults (press <F5>).
4. Insert the USB Flash Disk containing the BIOS file P5BDxxxx.ROM. (Note that the USB Flash Disk has to be inserted before boot to enter BIOS, otherwise it won't be detected).
5. Save and Exit <F10>. (BIOS will now restart).
6. During BIOS restart, press <DEL> to enter BIOS Setup again.
7. Under Tools menu, select ASUS EZ Flash 2. It may take 20 seconds for the drives to show up. Select a drive in the left pane <Tab>. Use <Up> and <Down> arrows and <Enter> to select a drive. If you don't see the BIOS file in the right pane, check the other drives. Note that the drive letters are not the same here as in Windows and that no HDD's (NTFS devices) are shown here.
8. Select the BIOS file (e.g. P5BD1101.ROM) and press <Enter> to start flashing. Supervise the update, which includes the following stages: Erasing - Flashing (writing) - Verification. BIOS will automatically restart when the update is ready.
9. Disconnect the USB Flash Disk before/during the restart.
10. Perform a C.P.R. (CPU Parameter Recall) which resets the chipset. Power down, remove power cord from the PSU for 2 minutes and then power up.
11. Enter BIOS and load BIOS Defaults <F5> again.
12. Reenter your preferred BIOS settings - if you are using AHCI or RAID mode, don't forget to set 'Configure SATA as' to [AHCI / RAID]. Also remember the settings for the JMicron Controller, Audio and Memory Voltage. Check also that the Boot Device Priority is ok.
13. Save and Exit <F10> (automatic restart).
14. Press <DEL> to enter BIOS Setup and make a final check that all BIOS settings are ok. Save any changes.