After browing the betfair forum, it appears that some betfair punters have been the target of hackers.
(can't find that thread now

Probably someone providing a link - which some people go to and it downloads a keylogging program which basically records what you type.
If you do not have antivirus software and antispyware/malware programs, i would suggest the following free programs
AVG : grisoft's free antivirus program
lavasoft's adaware : free spyware/malware remover
I also use pandasoftware's free online antivirus checker. Can take a while, but worth the effort.
Hopefully no-one here's been affected so far.