I'll have PIII 500mHz in a few days. Is it difficult to replace it? What I have to unscrew? what to do to not damage anything?
Replacing the CPU in a 770 machine is a bit of a chore, and requires disassembling about half or more of the machine. But most of the pieces are large and as long as you work slowly and carefully keep track of which screws are supposed to go with which pieces, then it shouldn't be too bad. I would be prepared to set aside two hours or more if it is the first time you've done it. I have an incomplete set of instructions available here:http://thinkpads.kiff.ca/770z/cpu_upgrade
You can also use the Hardware Maintenance Manual that includes a full set of instructions on how to remove each part. My instructions above were intended to collapse all those instructions into a single set to make it easier specifically for people who wanted to replace the CPU, but I never got around to finishing them.
In my opinion, the trickiest part comes fairly early on when you pull up the keyboard and detach various ribbons that connect it to the motherboard. This is step 7 in my instructions above.
Also, after you replace the CPU, you can definitely have periodic issues with various weird corruptions in your display, so hopefully this will not make your current LCD problem appear worse.