I can't stress enough how many systems I see on a weekly basis that are full of malware and viruses, causing costly removal sometimes involving fees that are often more than the cost of a replacement. Many times the true damage is identity theft which will not show up immediately and has very long-term effects. Usually the root cause of vulnerability is using a PC that is not fully up to date. You have to be diligent in updating all the software on your system especially with operating system patches, web browser patches, current anti-virus and anti-malware software.
Having said all that, read this article for some tips on Safe Online Christmas shopping. The one thing I would add is many bank, and credit card companies offer (even PayPal) one time use credit card numbers which work great for online shopping.