Client machine is Windows 7 with UAC turned off?
If you have the new Windows 7 interface turned on, where the default is the search bar and not the "run" bar, you could do a search for "Appwiz.cpl" (Programs and features) and right click on the entry it finds and "run as administrator".
Alternatively, you can navigate to C:\Windows\System32\appwiz.cpl and right clik from there to "run as administrator".
Try performing your repair from there, as "administrator". You should no longer get the "access denied" message.
Possibly, your installation has something corrupted and preventing it from retreiving updates directly from the server. If that doesn't help, you can try to limit the quantity of old updates the machine keeps, in case one or more has gotten corrupted. This can be done through the SEPM console.