Hi Chris

Firstly is it a laptop or desktop pc - Laptops are notoriously difficult to get showing the correct colours often the only solution is to compensate for it if this is the case.

If it is a desktop pc then it is an issue of colourspace and profiles. 

Are you using PS set up to colour manage images ?
Do you assign colour profiles to your images ?
If shooting or converting the image into a colour space other than sRGB are you converting the web copy ?

It is difficult to name the reason and solution until I know what settings you are using in PS and how much if any you colour manage your images.

You have mentioned calibration - this is not a calibration issue.   Calibration is setting up your pc to show images how everyone else who has a calibrated monitor will see them. It does not affect how images are viewed between different applications as it way of defining how images are viewed in all applications. 

I have in previous threads given a basic set up to view images on the web and to use for basic printers in a simple colour managed way suitable for many home users. 

I am more than happy to expand upon this further for those who want to get the best out of their cameras and high quality printers but I don't have the time right now (on my way to work).
 
hth - the start to finding a solution is to look at colour management and how PS views images.

Richard.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html