I would say yes it's a fair bet they could wipe clean the harddrive, (and also have a look at what you have on there - do you really want to have to worry about that?)
I would suggest you take HDD out and copy all contents - maybe make a backup copy of your 'My Documents' and also make a full drive Acronis Image or similar.
Then delete all your personal stuff including email, addresses, surfing history etc. and don't forget to empty the Recycle bin and all Temp folders.
If you get a USB TO SATA IDE CABLE / ADAPTER like this one from Amazon click here you can hook up the laptops HDD to another pc via USB and sort the contents out.