Things I would do. Unhook the computer with a problem from any internet connection. This has the side-effect of also making sure there is no connection to your laptop.

You then want to make sure your lap-top is protected. If you have a virus program on the laptop make sure it is updated. If you do not have a virus program OR the program cannot be updated (subscription ran out etc.) then try the free version of this program.

http://free.avg.com/

NOTE: Many ISPs have a free virus protection package. Comcast, etc certainly do.

You DO want to take the infected computer offline and keep it shut down for now. You do not want to keep using the infected computer since you're giving the problem more time to create MORE problems or do MORE damage.

You DO want to make sure your laptop is fully health, that way you can use your laptop as the doctor for your other computer.

Laptop:

Make sure the laptop's antivirus is up-to-date/installed and run it. Turn on as much protection as you can (you can dial it back later). Run the anti-virus program. If it finds anything have the program fix it, then restart the computer and run it again. Do this until it says everything it clean. If it can't clean the system you have a bigger problem.

If you can give the laptop a clean bill of health you can use it to Google to figure out what is going on with the other computer and then download and burn to disc what you need to try and fix things.

Goggle is an amazing tool for figuring out if your comp is doing something it shouldn't be. One trick in searches is to search for phrases by enclosing a phrase inbetween quotes (e.g. "jericho-kansas, inc". What you want to do is look for something that the other computer is showing on the screen that isn't going to be very common. For instance if it is saying the name of the company asking for the credit card then something like "company name" virus, or "company name" adware should bring up site where people are asking for fixes.

It is very unlikely that you'll have to buy a new computer. Usually the worse case is that you'll have to get a new hard-drive and/or copy of windows (but usually even that isn't required). There are ways even when the problem is really bad to fix things, someone can hook up a clean hard-drive and system and use that to clean up the other drive, etc.

You may find that by simply not having that computer hooked up to the internet that it doesn't lock up as it cannot establish a connection.

I'm flying a bit blind here as I cannnot see the problem. Does it pop up completely on its own or only after you launch the browser, etc?

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Also, please keep in mind that you'll want to back up what you can ASAP. This also means as you go along. If you get that comp to the point where it isn't locking up, back up everything you can (probably on CD) and be sure to label each of those discs as 'possibly infected'. Then try to fix the system and if you get it clean back up everything again and toss the 'possibly infected' discs. Keep in mind that things can go wrong and you always want backups if don't want to risk loss.

If you don't have anything on the computer you want to save you should be able to just do a clean install, if you go that route let us know and we'll try to help.