Trying not to be angry here....
Your "computer guy" is either a dangerous idiot who proves the theory that a little knowledge is a dangerous thing or, and this is worse, someone who knows what they should do and can't be bothered to do it.
He should have installed all the service packs and updates for you, or at the very least informed you that he hadn't and told you to do it ASAP. Windows XP without Service Pack 2 is a huge security risk, there once was a time when the half life of an uninfected system was less than the time it took to download and install the updates to make it secure. That is to say that you had at least a 50% chance of getting "infected" before you could hope to prevent it.
You wouldn't dare catch a mechanic allowing a car to go back on the road with unsafe brakes and that is basically what this man has done.
I'm disgusted.
I know a fair bit about IT through years of hacking away at various systems. However even I wouldn't dare to sell my advice as I don't consider myself expert enough. And I either know more than this "expert" or he's exploiting people like you.
You need get SP2 on there ASAP and make sure that your anti-virus is up to date. Download something like AdAware (www.lavasoft.com/products/ad_aware_free.php) right now and check if your machine is clear. Viruses and malware are known to interfere with Windows updates.
Also given this guys track record of being a general numpty I'd also check the license of the version of XP he put on your machine. Does your machine have a little XP license sticker or is one on your install disks? Your looking for an XP license number. You're looking for something with five sets of five letters or numbers looking like xxxxx-xxxxx-xxxxx-xxxxx-xxxxx.
Then download something like this:
www.magicaljellybean.com/keyfinder/
Run it to find your XP Product key.
Does it match the number from your sticker/cd? If it doesn't let me know.
I know that when I walk into a car showroom/mechanics I'm at a disadvantage and the staff smell me like a shark sniffing blood in the water. I know nothing about cars and will believe whatever I'm told. So when I see someone treating those who won't know much about IT in the same way it really offends me and I try to do my bit to help.