If I just want to replace the processor do I have to replace the fan too?

Most AMD processors will come with a stock heat sink. As for the old one does it line up with the mounting bracket on the board and what size is it? If it does not or is too small then yes you may want to send it back for a different one. Most places accept "I bought the wrong one for my processor." as a valid reason fror returning a heat sink.

Well, I can't answer that with the amount of information you have provided.
So please, post your Motherboard make and model
Your current RAM specs: DDR, DDR2 or DDR3, brand and speed (800, 1333, 1600)
VERY important: your current Make, model and specs for your power supply. It should have a label on one side
that lists all of that. A picture of that label would be good, but at least post the info.
Make and model of your video card. Neither your old processor nor your new one have integrated video, and your video card may be the largest consumer of power in your system.

My initial feeling is that this will work, but to confirm that would like to see the info above.
If I may ask, how much are you paying for that new CPU? There might be better CPU's available for the same or less cost

AngelGryl, the specifications that HP provided already tell you what processors your very old motherboard will support: "NOTE; only AM2+ processors are supported with this motherboard."
You are trying to buy a cpu designed for an AM3 socket, and though there are some mobo's in which that AM3 CPU will work (usually after some critical tinkering with the BIOS - which if done wrong can Kill the mobo) it may not function well with your memory (you have DDR2 and the AM3 normally uses DDR3) and HP themselves have told you their mother board will only work with AM2+ CPU's. Look at the list they have provided; it tells you what will work.
For the most part you will have to shop ebay to find those old CPU's as they are not in production anymore.



Sorry, but this doesn't tell us very much. Please look at my request for information: YOu have provided perhaps the most significant piece of info; that you are trying to upgrade an old pre-built HP machine. It can be done, but is it ultimately worth the expense? If this is your first adventure in upgrading/building we can try to help, but experience has shown that simple upgrades usually are not simple; upgrade one part, and you find you have to upgrade another, then another, etc. to get the PC to run stably. Don't know where you live, and can sympathize with budget problems, but it might be simpler, less frustrating, and overall cheaper to just buy a new inexpensive PC.
If you can find one of the CPU's that HP listed, then the upgrade path IS simple and should work. If you can find one new and a boxed edition, it will come with heatsink, fan, CPU, and the Thermal Interface Compound (thermal 'grease' or TIM) already on the heatsink so installation will be simple.
If you end up buying a used CPU or an OEM CPU - that is simply the processor - no fans, heatsink, etc.- your old heatsink Might work with it (it should fit, but the question will be whether it can conduct away enough heat) and you will need to purchase some TIM (I recommend Arctic Silver 5 or Tuniq 2 or 4). Here's a link showing how to properly clean the heatsink & cpu and apply the TIM. (It looks weird when you open it, but checkout the article)
http://www.tomshardware.com/forum/333968-28-thermal-paste-removal-installation-guide

Sorry for the long discourse, I hope it helps.

GREAT! Glad you found the right CPU, it should work well.
You might try contacting HP or even AMD to see if they will either send a new fan or sell you one for a reasonable price; that way it should match the heatsink. Few years back I thought the fan had broken on my Phenom heatsink and AMD not only replaced the fan, they sent a complete new heatsink and fan. Hell, it's worth a try.