Freeze at welcome screen

Back in June 2010, I put together a new desktop. It was my first time putting a computer together myself, but it seemed to have turned out okay. Barring the occasional issue with freezing a few minutes after leaving sleep-mode (requiring a cold reboot via the power outlet in order to get any response), the rig worked beautifully. The system specs are as follows:

• CPU - Intel Core i7-920
• MB - Asus P6X58D Premium
• RAM - Corsair Dominator 6 GB (3 x 2gb) TR3X6G1600C8D
• GPU - Nvidia GeForce GTX465
• HD - Western Digital 1 TB Caviar Green
• PSU - Corsair TX650W
• OS - Windows 7 Home Premium 64bit

About a week ago, however, I booted up to find the computer freezing on the welcome screen. Suspecting a recent Windows update to be the culprit, I restarted in Safe-Mode and restored the system to the previous restore point. This did not help, only causing the load screen to get stuck on a black screen with movable mouse cursor rather than the blue loading circle. While in Safe-Mode, the Windows Help window would automatically open but not fully load, eventually giving me an "Explorer had to shut down" error window, followed by an automatically repeated attempt to open up the help window again.

I checked to make sure I did not have a faulty memory stick by leaving only one of the three in at a time and trying to boot. I also ran the memory diagnostic tool from the Windows 7 DVD boot menu, which did not seem to yield any results. I also ran the 'System Repair' option from the same menu, but after the requisite restart, it would just leave me stuck on the same screen.

When I noticed that the system would also get stuck on the load screen for 'Safe-Mode with Networking', I thought that the networking drivers might be corrupted and disabled all of them in Safe-Mode. This allowed me to log in to 'Safe-Mode with Networking' and normal mode, but they both ran really slowly and unreliably. I was able to enable the network drivers and connect to the internet from normal mode, allowing me to check that all my drivers and Windows were up to date. I also made sure that my system wasn't fragmented (though it wouldn't be, with less than 15% of the disc used and defragged at least monthly) and ran CCleaner and the Windows Security full scan, with no results. After restarting with the network drivers enabled, the system once again started freezing.

I am at a loss for what to do, as I am by no means an expert, and I don't want to reinstall Windows because there is quite a bit of data (namely irreplaceable family photos) and program installations (with saves) that I don't want to risk losing, and I can't find an option to reinstall Windows over the previous installation with no data or program loss, as used to be possible in XP.

Any help would be greatly appreciated.

Thank you for the reply and the suggestion, Golden. I have already scavenged what I could fit into an external hard-disk. After running SFC as you suggested, I got a report that not everything could be repaired. The problem persists. I can attach the CBS.log file if needed, as it is all Greek to me, but it seems to date back for months.

I doubt that it is a hardware issue, as the information on the hard-disk is still accessible and the memory has been checked two different ways. That's not a sure thing, of course, and I don't know how to check the CPU, but I feel that it is probably a software issue relating to some driver or Windows update. Either way, though, I have no idea what to do next but want to try everything possible before having to wipe the slate and reinstall Windows to see if that fixes the problem.

So, any other suggestions, whether in regards to how to use the SFC log or in general, would be very welcome indeed.

Update:

Whether it is due to my having run SFC or simply gaining enough patience to sit through the loading screen, but Windows does load in normal mode with networking enabled. It simply takes forever and a day. Pressing Ctrl+Alt+Del and opening the Task Manager helps get past the black screen with a cursor, but everything still reacts so slowly as to make doing anything completely impossible. Safe Mode is also slow, albeit faster than normal mode. I tried doing msconfig and disabling all unnecessary startup and services, but that did not seem to have helped. I also tried downloading CPUID to check my CPU, but there was nothing out of the ordinary as far as I could tell.

I've not really narrowed down the problem, but I am flying through my admittedly limited set of known solutions.