I am an arranger for a high school marching band and I have just purchased Virtual Drumline 2 and Sibelius 4...
I use both to accurately gauge what i've just put down in Sibelius 4 is what I want to give to the
student musicians...Also I used Sibelius 3 and Virtual Drumline 1 to create audio cd's for the players so that they have some idea of what it's supposed to sound like...I currently use a 2.80 GHz processor with 2 GB of RAM but with no sound card...
I am unable to get VD2 and Sibelius 4 to play without freezing my computer...
Is it because I lack a ASIO compatible sound card?
or like what other posts have stated: is it impossible to get uninterupted playback using
Sibelius 4 and Virtual Drumline 2?

It's certainly not impossible to get uninterrupted playback.  The 2 gigs you have on your computer should be enough.

Your computer is not freezing because of the lack of a soundcard.  My guess is that you've missed a step somewhere in the setup process.

However, without a sound card, you obviously won't be able to hear the sounds.

Right, without a soundcard, how are you hearing sounds?

Oftentimes, if you hear glitches in playback, it's due to your CPU (processor) becoming overloaded. There are many tips that can help you achieve smoother playback:

1) Adjusting the latency setting in VDL2's soundcard setup window.
2) If using Sibelius Kontakt Player Silver/Gold for your band sounds, use the ";eco"; set, and make sure reverb is OFF.
3) Load the ";Lite"; versions of battery instruments, vibes, marimbas, etc.
4) If you have a good amount of  RAM (which it sounds like you may), try deactivating DFD in the VDL ";Options"; window.
5) Streamline your machine using the tuning tips at http://www.musicxp.net
6) Use ASIO for your audio interface (soundcard that uses ASIO drivers is required).

Hope this helps.

Can you define ";freeze"; for us?  That could be so many things...  please be our eyes and ears for us.

Could a usb sound card solve the problem?