Free security software from your bank
Some banks will offer you internet security software for free. Barclays customers, for example, can download Kapersky's internet security suite software, which includes anti-virus software and firewall (worth £51) for free (this can be installed on up to three computers in your home).
And HSBC's customers can download a free 12-month subscription to McAfee Security software here. Note: both of these offers are only available to PC users.
Free anti-virus software
If you can't get protection from your bank, you can download very good, free anti-virus software for Windows and Linux from AVG or Avast (note: this is only available for non-commercial or home use) but you must ensure to keep up-to-date by downloading the updates.
You could also consider Ad Aware which is a free program that that detects and removes malware, spyware and adware on your computer.