1. Make sure your powersupply can deliver a minimum of 13,5V when programming(erasing). 12V is too low for some pics.
2. Make sure that your COMport is not sharing IRQ with anything else.
3. Make sure your programmer works with another type pic like PIC16F627.
4. Try changing the programming speed(high,normal,low).
5. If you're doing ICSP, try programming the pic directly in the programmer with no ICSP cable attached.
6. Try another 12F629, just to make sure you didn't fry it on your first testrun.

/Ingvar

PS. Don't forget that SW5 should be in the "PROG" position. DS.