If your warranty is expired, Sony will not fix it for free (they did for a short time, but not anymore) - $45 to fix it or $75 for a refurbished PS2. Phone # for sony: 1-800-345-sony
If you're getting DRE or game freezes and your warranty is expired:
1. Try cleaning the game disc.
2. Try standing the ps2 vertically (my ps2 would freeze quite often with Kingdom Hearts' cutscenes and this did the job)
3. You can try a cd player cleaner kit but I'm not sure how effective those are.
4. Open the PS2 up, it's quite easy. You'll need a phillips screwdriver and one that would fit tiny screws. Just be somewhat careful when taking off the top part of the casing since the eject/power buttons are connected by a ribbon to the system. There's a lot of slack at least two inches but as always take your time. Then clean the laser lense with q-tip and water or alcohol.
5. Mark the initial spot of the white cog and then just keep testing the different positions. You can also adjust the tilt of the laser by tightening loosening the little black screw that's holding the laser to the rod. Use a white crayon and make an initial mark.
website for the above steps: http://arstechnica.com/articles/paedia/hardware/ps2.ars
6. If all that doesn't do it you can adjust the resistance/voltage for the laser. You've really nothing to lose since a PS2 w/DRE is essentially a giant paperweight otherwise. Make sure to unplug the system. It's good if you have a multimeter (you want to measure the ohms of resistance). Unscrew the two screws that hold the rods (that the laser sits on) down. The laser is still connected by a ribbon but you don't have to take it off; just flip it over and you should see two screw like things. These actually control the resistance thus the voltage that the laser receives. If you're having problems with reading CDs (ps1, ps2 blue discs) adjust the one on the right. For dvd's it's the one on the left. I did this (even without a multimeter, I just turned it ever so slightly) on a DRE ps2 that couldn't read dvds and it was successful. Now I have a working PS2 that I bought for $25 in my room and one I already owned in the living room. Sweet. Found these two very helpful sites with great pics: