Think it to do with the language settings, you are on English (US) when it should be English (UK).
I can't for the life of me remember how to change it though.
*edit* found it...on another forum....
Right, assuming Windows XP, there are several things to check.
1. start/control panel/date-time-regional options/regional options
select English (UK)
select location (UK)
2. click on advanced and select English (UK)
On the task bar, there is a square "EN", Right-click this.
then click settings
select default input language English (UK)
3. In the installed services box, click the United Kingdom keyboard, and then OK, and restart your computer.
If there is not a UK keyboard option, you will have to click the ADD button to add it.
This last bit always catches people out.