Don't allow it to run too long without AV and other protection. Using a firewall is a matter of preference, the router one will give you almost 100% protection from threats coming in, but can do nothing about them going out. If you pick something up from an email or removable drive, your machine can be compromised and you will know nothing about it. A software fire wall can help prevent this. These are good free products which do a good job of keeping a PC clean.





Another useful tool is Protection from Adware, Spam, Viruses, Online Scams | McAfee SiteAdvisor
which alerts you to dodgy sites.