Thorn, make sure there are no pictures saved on ur PC before sending it back to be repaired. If they find evidence of your grow, they may well (and they may be obliged by law) to pass that information onto the police. Backup all photos onto a CD and then delete them from the machine, making sure you also empty the recycle bin as well.
Most staff will not be bothered about it and will do the repair and take no interest in what is stored on your machine, some will take a look through or even search for .jpg .bmp files or text files either because they are nosey, or out of a genuine wish to try and save some of your personal files for you to make you a happy customer and get them some extra brownie points with the customer and their manager for 'going that extra mile to ensure the customer is happy'.
Better to be safe than sorry, I always used to tell the customer I would try and save any files they had and put them onto a CD for them, so they knew if they had anything they didn't want me to see they could remove it (mind you some didn't and you do get to see some interesting photo's of their wives, and thats always a tricky one, do you copy them to CD, and the customer then knows you have seen it, or do you not bother and pretend that you must have missed them?).