For the last 2 years I've been using a Dell Studio XPS 16 (with Windows 7) and in terms of image quality it's far and away the best computer screen I've ever used. Although this model is no longer available to buy new, I'm sure there are comparable models. I wouldn't hesitate to buy the same again, although it does have some drawbacks.

Specific pros and cons are as follows.

Pros

1. The screen is LED-backlit, as opposed to the older fluorescent lamp backlit style of LCD. This makes for a brighter, more even backlight that doesn't degrade as the display ages. My last laptop's screen (fluorescent backlit) was very dim after a couple of years.
2. Moreover, the backlight uses RGB LEDs: clusters of red, green and blue LEDs rather than white ones. This produces a wide gamut display with excellent contrast and colour reproduction.

Cons

1. The wide gamut display has to be properly calibrated, and using a calibration device with explicit wide gamut support. Out of the box it's pretty much useless for photo work because everything's way over-saturated, but once calibrated it's perfect. It just means you need to factor the cost of a Sypder 3 Pro or similar into your purchase. (See my answer to this question for plenty more information on this.)
2. It's a 16:9 ratio HD screen (1920x1080). I'd prefer something squarer for photo work but I soon got used to it and regardless of ratio it's still a nice big screen.

No other device I've used (other laptops including a brand new MacBook Air, iPads, phones, etc.) come close to this screen in terms of colour reproduction. That's not a PC/Mac thing, it's an RGB LED thing. Once you've used an RGB LED screen for photography work there's no going back!