You'll want at least two USB sticks. One for booting off of and one to save your stuff to.
The easiest system to get it done quickly would be Ubuntu. Download it
here.
If you use a computer with Windows 7 or later you should be able to just double click the .iso file to 'open' it.
Erase all the files on the USB stick and copy the Ubuntu iso files onto it.
That's all you'll have to do on a different computer.
Then we'll go over to your desktop and turn it on with the Ubuntu USB stick plugged in. As it boots try to see if the splash screen on boot mentions pressing a key to reach the boot menu or BIOS settings. Boot menu preferred. (If it doesn't say what key to use, first try the Delete key {Not the backspace}, if not that try F7, then F2, then just try mashing all of them at once to see if any of them work)
During the splash screen mash the selected key a bunch of times until the menu pops up. If the windows boot starts you'll have to shut it down and try again.
Once on the boot menu, select the option to boot from USB. If not that, try selecting the device that sounds the most likely to be your USB drive (It will likely mention the brand name). If you get the BIOS menu instead, search around for the option for 'Boot order' and set the 'USB Boot' (Or the device that sounds like your USB drive) to be the first option. This is probably the most involved step.
Once you have that done, the Ubuntu system should load up and prompt if you want to install Ubuntu. Just ignore that and and click the 'Try Ubuntu' option instead. This will load up an instance of Ubuntu that is not saved to your system.
Once it loads up, you should be able to open Ubuntu's file manager. Find your desktop's hard drive in the list on the list to the left side. From there you can access all the files that are on your desktop's drive. Plug in another USB stick and just drag over the files you want to rescue.
Once you're done with this, you can follow
One of many guides for it to go about reinstalling Windows itself. Just make sure you have your Windows activation key, which should be attached directly to your computer as a sticker somewhere.
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