I have been looking for a way to sync my files (on my Windows XP machine) to the cloud and at the same time sync those same files to my external hard drive.
Is there any software out there (pref free / open source) that can accomplish this task?
A program I use is Allway Sync which will let you do this. It's free with a limit of 40,000 files it will sync every 30 days. It doesn't cost too much ($19.95 with the offer today) to upgrade to the pro license which removes the restriction.
It lets you sync to multiple locations at once and there are loads of options for automation. It also supports a lot of the popular cloud services out of the box (Amazon S3, OffsiteBox, Google Docs, SkyDrive, iKeepinCloud, Dropbox) and can also upload to an ftp server.
If you want to use it to keep files backed up it also has the options to keep a certain amount of previous versions of files.
Another piece of software that you might want to look at which is completely free is Microsoft's SyncToy which I'm sure you could get to work with a bit of fiddling.
I've used Allway Sync for the past few years now and it's been working perfectly so I'd recommend that. I'm sure it will do what you need.