There is lots of spyware, malware, viruses, trojan horses and even keystroke loggers out there that make this type of intrusion more and more common. I just spent 3 weeks fighting and finally deleting a nasty pair of Trojan Horse malware "bots" that got onto my wife's computer while I was using it (the HDD on my laptop had crashed two weeks before and I was gettin git fixed). It took a combination of at least 10 different specialized programs

to find and remove these buggers, including getting deep into the Registry and running full scans with special detection software. Why so tough? Because the malware migrates

and clones itself into multiples. This pair were unpleasant, but probably not as malicious as some out there. We've checked our CC accounts (I only use one for any Internet transactions and then only on reputable sites.)
My advice? Use a good anti-virus program (there are several very good free ones that are much better rated than the big name ones. I use
Avast antivirus (free from Avast.com for home use), but
AVG and
avira are two othere that are highly rated and free. The best part of these programs is that they update frequently and have excellent databases. Anyone that doesn't run good antivirus software is vulnerable to attack from lots of sources, so practice safe surfing!