Additional information required! What motherboard do you have? You can open the case and check or use CPU-Z to find MB model number. If your computer is OEM built (depending on who built it) you may have to contact the manufacture to find out supported CPU’s . Truthfully I would not invest any money into that computer. Save for a new one. If your board is a socket 423 the processor prices are higher and there is a smaller selection of speed bins than socket 478.
The issue that you have is that the 845 chipset only supports Intel® Pentium® 4 processors with a 400 and 533MHz FSB in socket 478 (Northwood core). These processors have been discontinued for 7 years. Here are a couple processors that may work on your board http://ark.intel.com/compare/27442,27444,27446,27447
In the end you would be better served by going down to Walmart and buying their cheap desktop then to try to find one of this and think that you are going to get to much of a preformance upgrade out of it.