I finally got my 12F629 working.
I googled for hours but didn't get any instant solution.
Later i read that VPP should be applied before VDD (from the datasheet).
On this basis, i modified my programmer using only one more component.
This enabled my old programmer, which has no VDD control, to program the VPP before VDD chips.
Its a really simple solution, but so many people are spending so much cash buying new programmers and stuff. I Don't understand why people don't look at the datasheets.