[Addendum to previous posts regarding the 'beep test'...]
You say that your desktop starts but nothing comes up on the screen... if this is so, then how do you know that your desktop has *actually* started? Can you tell us exactly what you see? Is the screen blank, but you're assuming that the desktop has loaded? Or does the desktop literally load, but something unexpected happens?
Assuming that the type of memory you've purchased is compatible with your motherboard: find your motherboard manual (if you even got one with your machine - which is unlikely if you bought if off the shelf), or assertain your motherboard model and find a way to download the manual (it'll likely be in PDF format).
(If you're not sure whether the memory you purchased is truly compatible with your motherboard, skip to the last paragraph. Otherwise proceed with CARE.)
Now, within seconds of your machine starting up, a screen will appear offering you the option of entering the BIOS (it may ask you to press DEL, or some other key, to enter the settings). Do that, but only that. Now, turn to your manual and look for the section that determines RAM timings in the BIOS. It should detail the options, yet most options should have a 'default'. Proceed to the relevant section of the BIOS and change these settings to default, if they aren't already. If they are already at their defaults, manually select the lowest setting in each category. (For the category that offers T1 and T2 as options, select T2.) Now, return to the main page of the BIOS and save the settings.
Be warned, though, incorrect BIOS settings may harm your physical hardware. Further more, your machine may fail to restart and just sit at a blank screen. Should your machine fail to restart because the changes you made to the RAM settings were incompatible with the installed memory, turn off your machine and remove the watch battery from your motherboard for 30 seconds then reinsert it. This will cause the entire BIOS settings (not just the RAM settings) to reset themselves to some factory default values. These are usually safe values, but your BIOS may have previously been customized. So, it may be a good idea to go through each section of the BIOS and note down what settings have been set prior to changing them - a lengthy and dull task, true, but it may be worth the while.
If nothing here solves the problem - or, alternatively - update your question detailing the motherboard make and model *and* the RAM make and model (including timings labelled somewhere on the RAM in the format 'n-n-n-n' were 'n' is a number) and you'll likely get a more productive response.