Highrez

XMBC Version: 2.18.8
Windows Version: 7
Mouse Information (brand/model): just a cheap mouse
Relevant Computer Information (CPU, RAM etc):
Did the problem occur after an upgrade of XMBC? (If so, from what version?):
Did the problem occur after a Windows update/upgrade? (If so, from what version?):
How long have you used XMBC?: Maybe 1 year
What language and keyboard layout do you use in Windows?: English UK

Clear description of the problem - try and include as much information as possible, including what button and mappings you are having problems with (if applicable).: i want to hold 'w' and repeat shift at same time

Possibly, but I'm not sure it will work with just shift - not something I have tried..
In sim keys, select method 6 and try:
{OD}Z{OR}{SHIFT}

You may need an {OU}Z at the end - I cant remember (probably not but if the above doesn't work, you could try it)
{OD} only sends the following key when the button is first pressed down.
{OR} only sends the following key when the repeat runs
{OU} sends the following key only when the button is released.

The only problem, as shift is a "modifier" I'm not sure it will do anything on its own!

@phil & @Zeinn
Shift won't activate on it's own in this case but I got around this problem by simply doing {shift}p
(or any other random key) Meaning... I just added some key that didn't do anything to my game so that shift would still activate.

OK, I will have to look at the code to see if there is a way of triggering just a shift key (without it acting as a modifier for the following key) thats how {SHIFT}{CTRL}{ALT} etc are normally used so I guess it may be assumed....

I hope I don't sound like an asshole with this suggestion, but for the time being, you can perform that function easily with AutoHotkey. After installing Autohotkey, right click, make a new script, and paste this. Then right-click on new file and hit "Run Script".
X Mouse and AutoHotkey are a lethal combo.