I would like to use the editor that opens in PS CS5 when you open a Raw file.
I would like to use it on a jpeg file as I find it extremely useful to get the right colouring.
Is it possible?
Yes, it's possible. I use it a lot.
Open the settings for Camera Raw:
In the JPEG and TIFF Handling section at the bottom you can change the setting for JPEG to "Automatically open all supported JPEGs". Now when you open a JPEG file, it will open in Camera Raw.
I think you're referring to Adobe Camera Raw. If so you can use this same interface for JPEGs but you have to use a round about method to do it.
The only way I've found to open JPEGs in ACR is to use Adobe Bridge, right click on a JPEG and select "Open in Camera Raw...":
The white balance controls will behave slightly differently (you specify relative rather than absolute values) but I still find it a very useful way to edit JPEGs if you're used to processing raws.