There seems to be a mini-epidemic of this kind of problem.
Someone suggested in another forum that for Cakewalk sequencers to work, you must install the software with the same user name that you log in to Windows with. I'm not sure about that at all - seems like Snake-oil, but you never know. I use the same name and have never had the problem.
However, what I suggest, if you really want to use your Windows card in Sonar is this.
Start Sonar
Continue past the error message.
Sonar menu Options/Audio
Advanced Tab
Check "Share Drivers with other Programs".
Close Sonar and then Reopen it for the change to take effect.
To remove the Windows Soundcard from Sonar - which is recommended, it will reduce your audio performance.
That Options/Audio Menu again.
Drivers Tab
Click a channel name to turn it Blue background (enabled, Sonar will use it)
Or click until it turns White background (disabled, Sonar will not use it).
Once that's settled, I recommend you run the Wave Profiler again. At the Bottom of Options/ Audio/ General tab, pull the latency slider all the way to the left and click the Wave Profiler button.
Disabling Audio drivers does not prevent you using any midi synth or midi i/o port the card has. You select midi in the Options/ Midi Devices menu in the same way.
BTW, If Share Drivers does not work - there is something badly wrong with the drivers for your old soundcard (they may be working, but they are not "multiclient") and you will have to deselect them in Sonar anyway.