Anyone using a computer on the Internet. particularly for business, should do a 1-2-3 security check
Updates / antivirus / firewall
Make sure you are getting the latest security updates for your software, mainly Windows and Internet Explorer, or other browsers such as Firefox and Opera.
Ensure your anti virus (and anti spyware) programs are up to date and working. It is no good having one that came with the computer 3 years ago, if you haven't paid a subscription to update it.
Firewall - make sure you have on - either Windows own one, which should be switched on by default (check in Control Panel), or get proprietary one from another supplier
If your computer is not secure, it is like leaving the front door open to anyone that wants to come in and help themselves.
Do an audit of your equipment, particulalry if you are running a business, you shouldn't be using a 7 year old computer, or one you acquired from someone else, or even the home PC that the whole family uses to downloads all sorts.
Wireless networks: if you have one make sure it is secured (encrypted) there are plenty that aren't! Look in the wireless connection utility (in Windows) and see what the name of your network is (don't use your own name) and whether it says 'Secured'. If not, get it secured asap, or someone else could be using it, or even getting into your PC!