MS Antispyware Beta 1A was set to expire on July 31, 2005. Shortly before it did, Microsoft released Beta 1B, which fixed a few glitches and extended the "expiration" date to December 31, 2005, at which time they should release Beta 2. However, you must uninstall your current copy of MS AS, then download the latest version from their website...it won't update automatically.
I had no problems installing Windows XP SP2, and as far as I know, there's no real trick to it. However, since you mentioned recurring problems with trojans, I'm inclined to point to infiltration as the cause of your internet woes. Thus, I ask what you do for a firewall. While Norton Antivirus 2005 helps remove viruses/trojans, a firewall acts like a first line of defense, preventing such invasions to begin with. (It's like leaving your front door unlocked and open, but then carrying around a steak knife for protection...it's does more harm than good.) I'd suggest downloading and installing ZoneAlarm, a free firewall which provides excellent protection, but basic features. You can also upgrade to the Professional version for more advanced protection.
Finally, since you said the trojans keep coming back, I'm gonna guess that they never left your computer in the first place. First, you must disable System restore, which is where they like to hide secondary copies of themselves so they can reinfect you after your run a set of scans. (NOTE: This will erase all of your restore points.) Right-click "My Computer," select "properties," click the "System Restore" tab, and check "Turn off System restore on all drives." After that, restart your computer in Safe Mode by pressing F8 during boot. (A lot of viruses/trojans/worms will try to hide during a scan, or prevent you from doing one. By starting in Safe Mode, you'l prevent the critters from running, thus increasing your chynaces of completely removing them.) Once in Safe Mode, run all of the scans again and manually delete any files you know of. Then restart your computer and turn System restore back on (by unchecking that box). If everything seems OK, try reinstalling SP2...it should work without a hitch.
Hope this helps,
John