I'm really tempted to purchase Positive Grid so I gave the demo ago and so far really impressed with the sound quality.
The only problem is when I hit my guitar strings there's a second delay before the sound comes out of my pc making it impossible to play.
I'm using an old Line 6 UX2 for input. Could that be the issue?
Are you using ASIO driver? In Audio settings make sure 'Audio driver' is set to an ASIO option. Then, try playing with the 'Audio buffer size' option to reduce the latency. Some devices have a separate program you need to use to adjust this latency, so if that doesn't work you may be able to change it using a line 6 option, but I'm not familiar with this device so you'll have to research that on your own..