Talk to your repair company. As others have said, removing the hard drive is the only way to be certain that your data won't be modified or accessed. Although the work being done mainly involves hardware, having no hard disk may inhibit the repair company's ability to check that their work was successful. Then again, they may have ways of working round this (live CDs, for instance). Discuss their solution and how confident they are in it solving the problem with them.
As a compromise, there are some things you can do. At the lowest level of security, you could add a second (non-administrator) user account to your labtop and use your operating system's file permissions to prevent the second account from accessing any of your files. Only give the repair company the password for the second account. This won't prevent a concerted attack, but it will make idle snooping unlikely.
A more secure solution would be to encrypt any sensitive files with strong encryption. Either encrypt particular files (for instance, your address book) or your entire account. Again, only give the repair company the password for a secondary account.
The final, but most tortuous, solution would be to completely erase anything sensitive from your computer - again, either particular files or wipe the hard disk and put in a clean install of the operating system. Then when your computer comes back from repair you restore your files from backup (you are making one as a matter of course, aren't you? ;-)).