You don't need to buy a new computer. Something has invaded your computer and you need to clean it up.

Here's my cleaning ritual; they all have a freeware version.

1- Most importantly, I hope you have anti-virus. Even though the problem you described doesn't sound like a virus; it is still the most important thing to have IMO. The one I use is the popular Avast. Just get the free version.

AVAST Direct Link

2- I use ccleaner to delete unnecessary files that your computer temporarily saves but never deletes; inlcuding cookies. This program is optional though. You can definitely skip it but here's the link anyway. Get the Free version as well. Oh, if you choose to install it, you may want to uncheck "cookies" if you don't want to have to input your login information (such as pherotruth) after every cleanup. Although, they do have an option where you select which cookies to never delete; that's what I do.

Direct link

3- This one has been suggested above "Malwarebytes Anti-Malware" , the free version. No need for direct link as you have already downloaded it.

4- This one is against spywares. It's called "SUPERAntiSpyware (the free edition)".

Direct Link

So install these software, make a quick-scan with each, one by one. Hopefully it will remove it.

I like Mark's idea. After you go under control panel -> uninstall a program, make the list sorted by date and see which programs have been installed just recently. Is it something you have personally installed or is it a suspicious one.

Have you noticed any new toolbar that has has been installed on firefox?

And the days of "my Mac never gets a virus/malware/spyware" are long gone. Hackers hack that which is popular. Mac wasn't, now it is.

Shadow x pretty well got every step you need to clean it out, may also want to disconnect from internet while malwarebytes is removing the crap. Reboot, scan again and then reconnect

Clean up solution often don't help much.
The number one thing to do is:
1. backup all your data to a external device or to the cloud.
2. Download Windows 7 or 8.1
3. Burn it to an external CD or USB (to usb use live usb creator)
4. Install Windows and choose to format all partitions
It should work as fine again as it was when you first used it ;-)

You can download and install Windows even free if you haven't an original disk. In most countries this is even legal. You find enough with youtube and google.

Riverce, first off downloading Windows when you haven't paid for it is NOT legal in any country, some may not respect or enforce copyright laws but that does not make it legal!

Second, Installing a fresh copy of Windows is fine, IF you don't mind the hassle of installing it and replacing all backed up data, but clean up solutions can and do work if you know what you are doing. Even if you get a particularly nasty virus that MalwareBytes can't clean, there are things like tdsskiller and RogueKiller that will find and remove them from your computer.