Hi folks - advice needed from you brainiacs.
 
I have 2 PCs:  PC1 - this is my 'music' PC - used mainly for Sonar and any music-related apps and plug-ins I've accumulated.  This PC currently runs Windows 7 home premium 64-bit. 
 PC2  - this is a workhorse PC for general internet use and emails, to contain general music files and i-tunes and other media players, Office applications, general stuff. This workhorse PC just crashed big time and the Vista Business OEM OS can't be reinstalled (the PC was a cheapie I got from a company who were re-equipping and Vista OEM came with it).  
 
What I want to do = 
I want to buy a full copy of Windows 8 on disc and install it on to PC1 in place of Windows 7 (ie NOT as an upgrade).   This is because I then want to take the existing disc for W7 currently on PC1 and install it on to the currently ailing PC 2 to get it back on its feet.  BUT - I don't want to have to reinstall Sonar and all the other music apps and plug-ins on to the music PC1 as part of installing W8.   Can I install W8 alongside W7 onto PC1 without disrupting the apps/prog files, then subsequently remove W7 to put on the workhorse PC2?
 
I did think I could just create a new partition and put W8 in there but it's a while since I messed with partitions/installs etc and as you can tell I'm fairly clueless.
 
All advice gratefully received, cheers.

It first depends on the Windows 7. If that was an OEM copy supplied with the PC then it will be licensed for use on that PC only. Windows checks this by reference the specific motherboard, network adaptor MAC address and maybe a few other things when you validate/activate it. It may or may not successfully activate if installed on a different PC.

Your best bet to put Win 8 on the Win 7 PC is probably to install it as an upgrade if possible. Boot into Win 7, load the Win 8 DVD, run it and follow the instructions. Installing a fresh Win 8 while keeping Win 7 is also possible, but your PC will then be set to dual-boot Win 7 and 8. Applications that were installed under the Win 7 installation will need reinstalling while running Win 8 for Win 8 to run them. The two Windows installations will have seperate registries of their own and their own system folders, in essence they'll be two completely seperate operating systems.

Personally I'd go for Windows 8.1 rather than 8, unless you have hardware that doesn't have a Win8.1 driver.

Definitely Win8.1 to roll with. It may be a bit of APITA, but.i would just bite the bullet and do a clean install of the OS and all programs.

Many thanks for these replies. That clarifies the position - it looks like I'll need to reinstall all the apps whichever way I go.

Combo
Many thanks for these replies. That clarifies the position - it looks like I'll need to reinstall all the apps whichever way I go.

Although it involves more time... a clean install of the new OS is the best option.