Ok, I'm going to try to try to actually answer OPs question. On Windows 7/Vista, a set-it-and-forget-it configuration that is secure. (I'm not going to give an alternate OS or a setup that will confuse grandma)
Use Chrome Browser. It has its own built-in sandboxing and is generally less prone to viruses than all other browsers. Configure plugins as click-to-play (chrome://chrome/settings/content). Disable all plugins except "Adobe Flash Player", "Chrome PDF Player", and "Google Update" (at chrome://plugins/). Disable Internet Explorer to prevent her from going back to her old ways. Set Chrome as your default PDF viewer.
Install an Anti-virus suite. No, not MSSE. You need more than just anti-virus. She needs a suite like Avast, AVG, or ESET. You need something that scans web sites, scans email, checks if software is up-to-date, and sandboxes scary downloads. Personally, I like Avast. If you use Avast, configure all the non-critical popups to only last 1 second, and turn off all non-critical sounds. It's free, but the Pro version is slightly better.
Set windows to "Always notify" on system changes. Viruses have gotten around Windows security to install into the system.
Set up a Standard User for her, rather than the default Administrative User that Windows defaults to. This will prevent viruses from installing into the system. It will also prevent her from installing system applications. She'll still be able to install some user-level applications, however.
Install Dropbox and tell her to always save her documents in the Dropbox folder. So, if the worst happens, she won't lose her work files. Dropbox also has a versioning feature, so she can recover old copies of files (in case of something like CryptoLocker).
Install BufferZone Pro. This will automatically sandbox Internet applications, like Outlook, Skype, etc. I like to exclude CHROME.EXE as Chrome has it's own sandbox.
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Ok, I'm going to try to try to actually answer OPs question. On Windows 7/Vista, a set-it-and-forget-it configuration that is secure. (I'm not going to give an alternate OS or a setup that will confuse grandma)
Use Chrome Browser. It has its own built-in sandboxing and is generally less prone to viruses than all other browsers. Configure plugins as click-to-play (chrome://chrome/settings/content). Disable all plugins except "Adobe Flash Player", "Chrome PDF Player", and "Google Update" (at chrome://plugins/). Disable Internet Explorer to prevent her from going back to her old ways. Set Chrome as your default PDF viewer.
Install an Anti-virus suite. No, not MSSE. You need more than just anti-virus. She needs a suite like Avast, AVG, or ESET. You need something that scans web sites, scans email, checks if software is up-to-date, and sandboxes scary downloads. Personally, I like Avast. If you use Avast, configure all the non-critical popups to only last 1 second, and turn off all non-critical sounds. It's free, but the Pro version is slightly better.
Set windows to "Always notify" on system changes. Viruses have gotten around Windows security to install into the system.
Set up a Standard User for her, rather than the default Administrative User that Windows defaults to. This will prevent viruses from installing into the system. It will also prevent her from installing system applications. She'll still be able to install some user-level applications, however.
Install Dropbox and tell her to always save her documents in the Dropbox folder. So, if the worst happens, she won't lose her work files. Dropbox also has a versioning feature, so she can recover old copies of files (in case of something like CryptoLocker).
Install BufferZone Pro. This will automatically sandbox Internet applications, like Outlook, Skype, etc. I like to exclude CHROME.EXE as Chrome has it's own sandbox.