With my Windows 7 build, I had to actually perform a repair, since updates were not installing correctly. I was getting random crashes etc.
Running A Repair Upgrade:
I was advised to put the installation disc back in and run an upgrade whilst Windows 7 is running.
It will essentially re-install all of Windows, but keep all files/programs intact (at least it should)
It did fix all the bugs I was having inc long shut down times/standby start up times, upgrades, etc.
So it must have been an unknown error in the installation.
Since you built this PC, you should have the Windows 7 installation disc, so all should be well
Just pop it in, run Setup, and run the Upgrade option.
After it has installed, make sure you have all your files intact. If some are missing (I didn't have any missing), its most likely means it has moved them to a destination on the hard drive.
You'll find them in C:\Windows.old or C:\$INPLACE.~TR and C:\WINDOWS.~Q (Make sure you have set Windows to show hidden files)
You can delete these using Accessories\System Tools\Disc Cleanup after the upgrade, by selecting to uninstall Windows components in the check box, as the files it backs up can be quite big in size; around 5GB I found.
Trust me and do it