System has been fine for nearly a year now so no idea whats caused this. I've run Memtest86 about 3 times in the last month and both RAM modules have passed 2 hours of testing.
Appologies for length of post, I am trying to cover all bases here.
Assuming that the following interpretation of what you say you did is correct, I will make the following comments - see below.
As I understand it, you physically removed both your power supply and graphics card from your system and then, after a day or so, physically re-installed the SAME PSU and graphics card in your system. After which you started getting the CRC error message each time you power up and the system fails to boot. My suggestions are based on this interpretation of what you say you did - if this is incorrect and / or incomplete, then you will need to clarify, so that any advice or suggestions made are appropriate.
Anyways, here we go. In the interests of completeness, I will give short derfinitions / explanations as I believe are appropriate. This in no way means that I assume you don't know what I am talking about, I am just covering all bases.
First: CRC stands for Cyclic Redundancy Check. This is a data transmission integrity check that is commonly used to determine if transmitted data has been corrupted. It is a useful tool, but it can be bypassed. CRC verifications are used in BIOS during booting, during data transmission over phone lines and during file reads from assorted media, including hard disks and CD/DVD-ROMS. It can also be used to verify RAM integrity. If you get a CRC error message it normally indicates that the data was corrupted to begin with, or the data was trashed during transmission. The second condition can occur as a consequence of either original data being NFG OR a problem during communication - which means problems with either the transmisssion process or the wiring. Most frequent case is faulty wiring / cables or connections.
Since you have run Memtest successfully, this indicates that both your BIOS and RAM are OK, as is either your floppy drive or optical drive. It wouldn't run successfully otherwise. This is good, as it reduces the trouble-shooting targets significantly. It also indicates that your MoBo has not been damaged.
Which leaves three items to look at.
Item one is the hard drive itself.
Item two is your video card driver files.
Item three is your OS installation.
Problems with all three can lead to CRC errors.
Lets start with a few questions.
1) were you able to remove the PSU without having to dis-assemble your system? Some case / MoBo configurations make it mandatory to physically remove most of the guts of a system to remove the PSU. You didn't perchance force things when you removed the PSU, did you?
2) If you were able to remove the PSU without dis-assembling your system, and without forcing things, I assume you took appropriate precautions to prevent static electricity damage. I am also assuming that you disconnected all power connections in as gentle a manner as possible - sometimes they get sticky and unco-operative.
3) When you were re-installing components, I assume that static discharge precutions were taken AND that you didn't overflex the MoBo and any other connections weren't disturbed, and that you ensured that all cables and connectors were properly seated. This is an accurate assumptin isn't it?
Assuming that you were able to remove the PSU without dis-assembling the system, that all appropriate static discharge precautions were taken, and that all connections check out you are left with very few checks.
Before doing anything else, check the following:
1) all cables to all drives remain properly connected.
2) your video card is fully and properly seated in its slot, and is stable.
3) every other component is correctly seated and connected.
Most likely is that the video drivers have somehow been corrupted. Try re-booting in "safe mode" and un-installing and then re-installing the video drivers. Very important: If you have the original CD for your video card, start with these drivers, as they are essentially guaranteed to work. Install any updates and patches you may have downloded after you are able to boot successfully using original drivers.
If this doesn't work, try booting in safe mode and verifying that the OS is not corrupted. It may be necessary to re-install and patch the OS. If this is the case, you may get lucky and not need to re-install your apps. And most drivers, but you should seriously consider re-installing the video drivers.
If this doesn't work, it may be necessary to reformat and re-install from scratch. Before you do this, you should verify that the drive is working OK. You should also back up any data files you need. I am assuming that you are applying the "one big drive" approach here. If you aren't, and have partitioned your drive(s) into seperate partitons, you won't necessarily need to back up your data, but it is still a good idea to do so.
You may want to consider pulling your HD out of your system and connecting it to a buddy's system that has something like Norton Utilities installed and doing some checks and repairs on it. And then backing up data.
You may want to consider running a very low-level check of your HD - Partition Magic comes to mind here and correcting any errors found.