I recently had a customer who got one of those "your hard drive is about to die" viruses. I removed the virus, fixed windows update that had been mangled, and thought he was ready to go. He called back though, and Microsoft Security Essentials would not update. It gave the error code 0x80248014. After doing some Google searches I came across a post on the Microsoft Answers website. Once I took these 4 steps it worked perfectly without even as much as a reboot.
Open a command prompt. We're going to turn off windows update service, clear the que and restart it.
net stop wuauserv
ren SoftwareDistribution SDold
net start wuauserv
Now try to update Microsoft Security Essentials. It should immediately update.