We really need to know what mother board or pre-built PC you are using.
Have you checked any of the settings in your BIOS since you changed your CPU? Did you clear the BIOS on your board after you changed the CPU?
If you have a PC that you bought already assembled, it is possible that the BIOS does not support your new CPU, or needs to be adjusted for that CPU, or the BIOS needs to be updated for the new CPU.
Have you ever seen the CPU temperature after you changed it? It is possible that an overheating CPU will be throttled to a lower speed. If the CPU cooler was not installed correctly, that would cause that problem.
I'm using an HP p7-1187c with a lot of upgrades (although still using the original motherboard). Here's the motherboard specs: Motherboard Specifications, IPISB-CU (Carmel2) - HP Customer Care (United States - English) .
I chose the Core i7-2600 because it was one of the few processors listed on the p7-1187c spec sheet under "Motherboard supports the following processor upgrades:". I haven't checked the temperature, but I would think that the heatsink that came with the processor would be good enough to keep it cool. All of the other fans in my PC are also functional, and my PC is in an air conditioned building.
I haven't messed with any BIOS settings - the BIOS does recognize the Core i7-2600, though. The HP BIOS is very basic, there aren't a whole lot of settings to change.
If you used the CPU cooler that came with your i7-2600, then you should be fine. You did change from a 65 Watt TDP CPU to a 95 Watt TDP CPU, so using the correct cooler is important.
You did not mention if you cleared the BIOS or not. That process is described in the page you linked to in your last post. It is also possible that the BIOS version you are using now does not support your new CPU. I think there is one BIOS update, but it did not mention anything about supporting any CPUs. The CPU replacement info did not mention anything about the BIOS either.
Does the BIOS display any system temperatures? It would be good to at least check once what the CPU temperature is, since you had to reinstall the CPU cooler. I use one called HWiNFO64, which you can download free and install, just Google for it, download, install, and run. If you have Windows 7 32bit, you should use HWiNFO32. You must click on the Sensors button if you don't run it in Sensor Only mode. It will also show you what speed your CPU is running at, and all kinds of other information.
Did you ask HP support about your issue? Did you check HP's support forums for any owners having a similar issue? It is very unusual to get a defective CPU, I've never had one that was.