Click on the little padlock in IE to be sure that you are doing what Bob said.
Do not click on links in emails to go to shopping sites- enter the web address directly.
Make sure your browser is up to date- older versions may have security vulnerabilities.
Also make sure you have no spyware that could steal your credit card details and send them to some crooks somewhere- run Ad-Aware, Spybot Search & Destroy and Ewido at least.
Make sure you have a firewall which allows you to control outbound traffic, and do not let unknown processes connect out to the internet- it may be a Trojan sending out your credit card details.
As 80% of computers are reported to contain spyware, buying online is a big risk if you don't secure your computer first!