What is a password log?
There is no password that will protect all the files on your hard disk. And if you want them to fix it they need to access the disk.
So basically, yes they will be able to access all the data on the hard disk, whatever it is.
Best thing to do is download a free product called ccleaner and run it.
http://www.piriform.com/CCLEANER
Select all the options on ccleaner (for all the browsers you run) and it will delete all your temporary files, history, and lots more besides.
Make sure you keep details of all your userids and password you use to log on to web sites as ccleaner will delete them.
Also make sure you remove ALL the personal files off your PC as some of the repair people are not that good at saving your personal files.
Buy an external harddrive if you have not already got one, and COPY (or better still MOVE) all your personal files to that.