When I was cleaning my CPU (with stock cooling), I broke one of the Push Pin Locks, I tried using it on three pins but the Temperature readings are going crazy, I tried looking for a replacement for these pins but I didn't find one. Does this mean I have to buy another CPU cooler, and if I will what brand do you suggest me ? knowing that my processor is Q9400 ? How to remove the old thermal paste from the top of the CPU ?
If you are comfortable with potentially having to take the motherboard out of the case to install a HSF (which you might not be if you have to ask how to remove paste), my default recommendation (if it fits on your motherboard and in your computer case) would be CoolerMaster's Hyper 212+ or 212 EVO.
As for how to remove paste: scrape what you can off with a plastic scraper (ex.: credit card) and wipe the rest off using paper towels and isopropyl (rubbing) alcohol.