They've got one of them binary closed source distros that shares the root drive with a weak administrative password. It's probably been hijacked by now and now the system is acting as a spam relay, or worse... Of course we can restore the file associations by renaming regedit.exe to regedit.com and then running that from the command line, changing the default association for files of type *.exe but that won't solve the underlying problem, you see, which consists of a collection of rootkits, bots, trojans, trojan downloaders, viruses, keyloggers and fun stuff like that.
If you're still out there, lissie82, I'm sorry to have to tell you this but it's very likely that your computer has some evil stuff on it. Downloading Spybot S&D and running that, also doing a thorough virus scan might grant you some temporary use of the machine, but I wouldn't trust it anymore for doing online transactions with your credit card. In fact, please unplug from the internet, back up your important documents to a writable CD and get one of your computer friends to reinstall your original factory disks. Tell them to "use a strong administrator password." Get a firewall.
Better yet, have them install a friendly linux distro for you, such as kubuntu. Linux is a free operating system that belongs to the community. It's nothing like the system that your computer came with, however. You won't be able to run a lot of software that you see for sale in the store--but on the other hand you can usually find free equivalents on the net that are compatible with linux. People often save a lot of money with Linux, but the tradeoff is it usually takes more time getting used to it, and I wouldn't recommend it unless you are willing to study, so it's not for everyone.