As I have a home studio, I use my desktop for general PC and Internet.  As others have mentioned, when I do run Sonar, I disable the Ethernet NIC and only have minimal running to avoid potential problems.  Could be I don't need to do what I do, but I feel better doing it.

I have a separate computer for the family, but...

I have a multi boot setup on my DAW PC. When I migrated to an SSD last year, I kept the previous Window 7 installation and I now use that for video and image editing. I also have another W7 boot for general use and I use it on occasions when the family computer is otherwise engaged. The main boot has nothing else installed other than what I need for audio production.

Thanks for the advice now I keep my new pc off the internet is it possible to install Plat and install all the updates offline?

I don't see a problem with multiple uses for your DAW, as long as you keep an eye on what resources the other apps are using. I would take a look at your startup programs and services and see if there's anything there that could potentially disrupt performance when streaming or processing audio. Some apps install things like background processes which check for updates and the like. Intuit software needs watching, and probably Adobe as well. For instance if you CTRL-ALT-DEL and look at your processes you'll see things like Adobe Helper and Adobe Update Service - it's always worth researching things like that and finding out if you really need them and if not, how to disable them.
 
Also watch for an app called f.lux which tones down your screen's brightness and warms the colors after sunset to help with sleep problems. Millions of people use it nowadays. It has a background process which continually polls the display to check whether other apps have disabled it, and this frequent polling causes audio glitches. I posted this on some audio production Reddits a while ago and was amazed at the response - hundreds of people saying wow, I was getting these glitches and I use f.lux and didn't even make the connection. It's an easy fix - you just go into the f.lux settings and select "safe mode." 
 
So basically it's perfectly feasible to use your DAW for other things as long as you keep a close eye out for problems, for which there are usually solutions. 

Multi-use here too.  I don't run WiFi or BT even though the machine has them.  I don't bother disabling e-net.  I never have problems with Firefox and Thunderbird running alongside SONAR, S-One or Reaper.

I built my desktop computer strictly to hold my DAW software (Sonar Platinum), plugins and soft synths and any other related audio software. I don't use it for anything else.  It is in an area of the house know as my studio and only I am allowed to go there.  ooohhhh  scary me !!!
 
For surfing the Internet or sending email or writing documents, I use my laptop or iPad.

BMOG
...I keep my new pc off the internet is it possible to install Plat and install all the updates offline?

 
Instead of using Command Centre to manage your installations, you can get all the installers from your "My Products" page and install them offline.
https://www.cakewalk.com/My-Account/Products

I have a dual boot system with 2 completely separate SDD drives.
 
1st drive is for gaming/surfing etc. it is loaded with ESET security, Spyware scanner and a sandbox utility. I do online banking when using this OS drive too.
 
2nd drive has SONAR on it and is much leaner, no security software or anything else just windows built in firewall. No surfing is done when using this OS drive.
 
This way I can have a dedicated system for SONAR and still use it for games and surfing safely.
 
Acronis back-up is running on the 1st drive which backs up both drives to a 1TB HDD...Now and then the backup folders get copied over to a 1TB external USB drive.
 

BMOG

Do you Use Your PC for more than just Recording

 
never.
ever.

My personal production, yes. For paying clients, no.