I work on Windows Server 2008 R2. This is very busy server. I use SAS on this server and often get 'OUT OF RESOURCES' message. I restart the server to solve the problem. I want to understand in detail that, what exactly happens when we restart the window.
Restarting a server isn't generally necessary unless you need a reboot after patching (which is a very common cause).
Your problem sounds like a memory leak due to a driver problem or something similar. You should diagnose your system and compare the freshly rebooted state to the state after a few days (using task manager, procmon, resource monitor, event log, ...). Look for process memory consumption, peak consumption, event log errors and warnings.
You might need to update system BIOS, device firmware and drivers or other software the system's got installed. It's generally a good idea to keep your system up to date - using a somewhat outdated OS is not an indication for good housekeeping...
Additionally, a "very busy server" might need moving off services to another system. It's a very good idea to use a specific server only for a single service (or a few smaller ones). To avoid buying loads of hardware using system virtualization is a good approach.