Reboot cycle after running chkdsk

Running Vista home basic on a Gigabyte GA-G33M/S2 with an Intel E4300 and 4GB of Corsair memory. I tried to install SP1 and it failed so I ran chkdsk to make sure there were no disk problems. It finished, fixed a couple of minor problems, and told me the disk was fine. Then it got stuck in a reboot cycle where it would get to the "Microsoft Corporation" progress bar, stay there for 5 to 10 seconds, then reboot again. When trying safe mode it would load some files then stop. After a couple of tries I got it going again with my Vista disk and a restore point (which I'm glad I remembered to make before I started). It seems to be running fine but I'd like to know what happened.

The "chkdsk" must have tried fixing something like your .mbr (master boot record) or some system files.
Or something went wrong and the Service Pack you installed corrupted something a system file or the .mbr or didnt install the way it was meant to, which I have found is quite common. Because of this the computer is stuck in your endless loop, do a system restore and try again.