It's a worm called Kak. I've had it before, it managed to infect the entire network. Now we have precautions!! (i.e. firewall/A-V). It attaches itself to e-mails, so anyone you have sent a mail to could be a potential victim. It also randomly appears to dissallow loading of windows on certain days. It pops up saying "Not today" or something like that then shuts down.

It's not particularly dangerous, just a pain in the arse. Get a A-V to clear it out. Make sure also to remove the signature section from Outlook.