Did you something specific when it popped up, i.e. browsing the web with what browser?
You could try to locate that string in the register, but be careful (Press Windows key + R key, type regedit and start a search); you may find more info there.
But I should surely run a full virus and malware scan, see:
Free Anti-Malware Detection & Removal Software
Furthermore you could use CCleaner, see:
Link Removed to check whether something unknown to you, is installed or is being started at system start up.
Henk