Only way for complete peace of mind:
Do not leave a copy of your Documents on the pc's HDD.
Get a USB external HDD and put all your documents onto it.
Check that they are 'safe and sound' and open okay from the ext HDD.
Make another separate copy of them just in case of mishaps.
Make an Image of your pc's HDD using Acronis or similar and store it on the external HDD.
Then if they mess up your pc you can restore it to how it was before they arrived.
Disconnect the ext HDD and lock it up somewhere safe.
Or/also you could restore the Image you made to a new internal HDD (then you will know image restore works ok) and remove your original HDD and lock it up with the ext HDD.
Boot the pc up with the new internal HDD and let them use that. Then if it gets virus or spyware infected you can just wipe it and restore again from your Acronis Image and pc will be up and running in minutes complete with all settings.
After they have gone home you can put your HDD and documents back on your pc and have no worries about messed up settings and viruses and spyware etc.