Last year I had to delete a virus file and was told to re-boot in safe mode to do it. I have wondered ever since why I couldn't just re-boot as normal. Could someone explain why to me please?

This document from Symantec tells you how & when to use "Safe Mode" & also why.
SAFE MODE

Thank you for your reply cswchan.

I know I'm probably being very dense, but I couldn't find on the site you linked to where it gives the reason that you need to re-boot in safe mode to delete a virus file. I read how to do it, and what it is. Am I missing something?

Here isa direct quote from the linked site.
"Safe mode is the Windows diagnostics mode. When you start the computer in Safe mode, only the specific components that are needed to run the operating system are loaded. Safe mode does not allow some functions, such as a connection to the Internet. Safe mode also loads a standard video driver at a low resolution. Due to the low resolution, your programs and the Windows desktop may look different than usual and the desktop icons may have moved to different locations on the desktop."
Safe mode is where all the extra stuff that normally runs on windows startup doesn't get run, making it easier for you to delete certain programs.... strictly because they're not running. Windows usually does NOT allow you to delete a running program.