Forget about porn, there are much more sensitive things on most of our computers - passwords, financial data, Social Security numbers, our addressbooks, enough for anyone to steal our identity, and perhaps our money!
The only solution is to use full disk encryption, and to back up regularly. If you hit a serious problem that you cannot fix, format the drive, send the machine in to be fixed, and then restore from your backup when you get it back. If the drive is encryprted and the machine too dead to even format the disk, don't give the repair people the password, and tell them for format the drive when they get the machine booting again if they need to.
At the VERY least you should keep your home folder encrypted, using FileVault on Mac, BitLocker on Windows, or an encrypted file system on /home on Linux. You should have a clean admin account on your machine that you never do anything with, and you only give the password for THAT account to the repair people. If you have all your home dirs encrypted, then all your personal data should be safe.
Another approach is to use external encrypted disks for all your data, that way, the really sensitive stuff is not on the machine. If you do this, be sure to use the portable version of FireFox and have it save your bookmarks, passwords, etc on the external disk.
B.