Dropbox is a great online service, giving you access to your stuff from anywhere in the world there’s a net connected computer. Use this link to setup a free account and download the Dropbox software. (Using that link increases my Dropbox space, just so you know!).
To backup your Presets, first create and then open your Dropbox folder. Create a new folder in this, call it something like ‘LR Presets’. Now in Lightroom, open Preferences (Edit> Preferences on PC, Lightroom>Preferences). Click the Presets tab. In the centre, on the right, there’s a button marked ‘Show Lightroom Presets Folder’. This opens the Presets folder. Now copy the contents to the ‘LR Presets’ folder you created earlier. Dropbox will then upload these to your account, so they’re saved online, and you can access them from any other computer when online via your Dropbox account.
Update: Victoria Bampton has suggested in the comments that you could setup a symbolic link between this folder and dropbox, for automatic backup. I’ve covered these links in the past in relation to moving the Previews folder. While this was a Mac post, there are Win related links in the comments of the post.
Update 2: Again from the comments: http://lifehacker.com/5154698/sync-files-and-folders-outside-your-my-dropbox-folder