These instructions are for firefox, however the same principle applies to IE and Chrome, however I do not know how to access the form history from those browsers.
Download and install this add-on, it provides you with a GUI to easily access sqlite files (databases)
Now if you are running Windows Vista or 7 browse to C:\Users\<UserName>\AppData\Roaming\Mozilla\Firefox\Profiles\<RandomString>.default\formhistory.sqlite
In xp it is C:\Documents and Settings\<UserName>\Application Data\Mozilla\Firefox\Profiles\<RandomString>.default
Hit browse & search then the search button. Now under the field 'fieldname' type in credit or cc and pick 'contains' from the drop down menu. Hit ok.
Now if you have ever bought anything on the internet with your credit card chances are all the information is saved here, full name, visa number, expiry, and CSV number.
Another way to check is to type in the first 4 digits of your credit card into the field 'Value' and pick 'contains from the drop down menu. Once again you will find your credit card number popping up all over the place.
Now I don't think it would be very difficult at all to write a small trojan that steals these databases and uploads them, certainly a lot easier to do than setting up a keylogger that has to run for weeks. Quite an easy way to steal some ones identity.
I recommend you delete all the entries you find, or even delete the entire file if you don't want any forms being remembered.
I imagine there are hundreds of sites that have been coded poorly and will cache these details.