Legally run IE for testing via Parallels on Mac, _without_ buying Windows?
Got it working. Microsoft currently publish free, official Windows virtual machines for things like Parallels and others, for exactly this sort of testing - currently distributed on http://modern.ie 


Go to the visualization page downloads section and choose 'Mac' then 'Parallels'.
For me (on Lion, July 2013) the "IE9 – Win7" VM didn't work at all, but the "IE10 – Win7" worked first time. Download it and extract the pvm file using the terminal commands provided. It's a big download, about 3.5gbs.
Run the newly extracted pvm file (after installing parallels) which should open in Parallels and boot up. If it asks if the VM was 'Moved' or 'Copied', chose "Copied".






A few tips for working with Windows through Parallels in this way:


Downloads within IE go to the regular Mac download folder by default
It seems to work first time with whatever input methods you have on the Mac. Copy and paste between applications "just works", and (unlike Virtualbox), if you usually use a non-standard input device like a graphics tablet, it works in the visualized Windows windows just as well as everywhere else
...that said, it still interprets the numpad default differently as by default numpad numbers are arrow keys until you hit the 'Clear' button (while your Windows VM is focused) then it acts just like when the NumLock key is on for PC keyboards, i.e. you get numbers. 
If you have trouble entering credentials for any web pages, it seems like it adds a domain of its own (e.g. "IE10win7") rather than using the default for your network. If you have trouble logging in, try adding your network domain with a Windows-style backslash (e.g. enter domain\username as your username instead of username)
To switch so that instead of ae Windows window with desktop, start button, etc, it treats each Windows application like a Mac application with its own separate window and doc icon, switch to 'Coherence mode' (View > Enter coherence)
In 'Coherence mode', where Windows apps are treated and docked just like Mac apps, if you need to get to the windows desktop view for any reason, use the red parallels icon in the tray at the top right and 'Exit coherence'.


And just in case anyone is in the exact same position as me, needing IE for testing and for doing stuff in Sharepoint, you can install Sharepoint Designer in the VM but it has to be the 32 bit version.