Update Errors and system rebooting-Error Code-0x80246005

I am able to veiw the list of updates, but it won't let me download and
install. I've tried the steps in past posts where I delete the Data Store
and Download folder, after stopping Automatic Updates, but it won't let me.
I suspect I have a mix of SP1 and SP2 files, but I'm not sure what to do.
This is what I have..

Winver - Version 5.1 (Build 2600.xpsp2.030422-1633: Service Pack 1)

Lsass.exe- Version 5.1.2600.1106
In the add/remove programs I have a bunch (17 of them) of Windows XP Hotfix
(SP2) updates, but Windows XP SP2 isn't in there. I looked because I found
a posting about removing and reinstalling XP SP2, but I'm not able to use the
updates. I can manually install XP SP2, but I wasn't sure if it was a good
idea.

In short, I have 2 problems. 1- Windows won't download the updates and
install (error code in subject line). 2- System is VERY unstable, it likes
to go on rebooting binges- I leave it for the night and it's OK the next day
(maybe). I'm really getting tired of this thing rebooting.

I've recently purchased new AV (McAfee 10) and it won't let me complete a
scan, I get errors EVERY time I try a manual scan.

I don't think it's a virus. Nothing found with MS Malicous SFTWR, AVERT
Stinger.

Please help...