It sounds like you have a serious registry problem. The answer to your question is "no, you do not have to buy a new computer." Howeever, at this point you most probably will need to pull out your Windows XP CD and clean install XP onto the hard drive and start all over.
 
If you can, backup your data. After this, if need be, get into your BIOS and set the machine to boot from your CD. Then, put your XP disk into the CD drive and clean install it. Be advised that this will destroy everything on your computer and once it does reboot and come up, you will have to download all the Microsoft fixes for XP, and any software programs that you use will likewise need to be re-installed.
 
Given the software that you mentioned, it is best not to mix McAfee and Norton. Both liberally substitute .dll's and modify your registry unconscionably. Having said that, it is your own personal behavior apparently on the web that caused you to turn to software solutions to either protect your machine from viruses, trojans and the like or to bail you out afterward once infected.
 
If you feel you can't break your habit of surfing areas that make your computer vulnerable after you have installed your security software like Zone Alarm, get a copy of Deep Freeze and put it on your machine. Installing it will allow you to return your machine to the way you had it configured after each reboot if it starts acting up.

Just a side note, seeing as you asked to use "dummy" talk. To get into your BIOS, you have to reboot your computer and hit the command associated with Setup. Usually, there will be a brief (and I mean brief) message displayed either at one of the corners or on the bottom of the screen just seconds after the bootup begins. It is usally one of the F buttons (e.g., F2).
 
You may have to do this a few times before you get it right.