Good advice there from BP.
For me, the ideal solution is to have 2 PC's --- One that connects to the internet for doing all the browsing and downloading etc, etc, and a second PC that is STRICTLTY for "DAW" work and nothing else. This PC has to be kept "clean" and hasn't to be prevented from operating at optimal performance.
Personally, I'd never allow my main Cubase box (DAW) to connect to the internet. For those who use XP, I'd recommend a "one-time" connection to the internet for O/S licence validation after your "perfect" build is complete. Don't do this until all your audio software is tested and operating to your satisfaction. After validating your XP, take it away from the internet and don't let it anyware near it again
Everything that reaches my main Cubase box (from the internet) has been downloaded first from a "general" PC (which has both hardware and software firewall protection). The downloaded files are virus scanned before transferring over to the main Cubase box. There is no network connection between the "general" PC and the Cubase box. Downloaded files are transferred using a USB2 external drive.
If you're not happy with XP and the O/S validation / licence internet connection "process", then Windows 2000 is a very stable platform for multitrack recording. Currently, I'm usng Windows 2000 because it's not full of all the "baggage" that comes with XP. That's something we can all do well without on our DAW's (IMHO). Windows 2000 does the digital multitracking job admirably and is WAY faster on my gear than XP.
There is another solution if you want more stability... Get a MAC !!!
Hope this helps,
Rab