not sure if my computer is secure???

Hi,
I wondered if anyone can help me, I need to be sure my pc is secure, i have xp. how can i be sure the person who built my pc does not have access to it remotly or something, is there any way they can do this without me knowing?
i have norton Internet sec. and AVG8 but could they change the user rights assignments settings / policys to give them access, how can i make sure they are all the correct settings. Also i have changed the computer name and the only user account is my own but it still says his name in the directory and in the sharing options i have unticked remote access but it says 'mike' already has access? (the person who built the pc)
another thing i cant install the security update >>Microsoft .NET Framework 3.5 Service Pack 1 and .NET Framework 3.5 Family Update (KB951847) x86

i know a bit about computers but not enough to know all this kinda stuff.
any help really appreciated here, thanks

 

ok done, thanks for that. Just another thing quick, How secure are my passwords if i get firefox to remember them?
sorry for bein a pain. i just have a lot of important accounts that i want to keep secure with all my personal data. I change my passwords like every week

 

If Norton Internet Security has an antivirus, in addition to AVG's, you should disable or uninstall one, probably AVG, as Norton is probably running your firewall also. Imagine Windows firewall disabled as well, don't want two firewalls running either. The best way to be sure would be to reinstall the operating system, if you have that option available. If you take that route a backup might come in handy, one of your drivers at minimum.
Firefox has the option to password protect your passwords, but I don't use the option to remember passwords in any web browser. Good idea to change them frequently, something I would bet most people don't do.
Possible resolution to .NET
http://forums.techarena.in/windows-update/1111697.htm

 

Paranoia strikes deep, doesn't it?