Depending upon which operating sytem (OS) you are using, you can check the available system resources by right-clicking on the 'My Computer' icon on your desktop and selecting properties. Under one of the tabs it will display the percentage of system resources, which are free.
Every programme, which is running is using system resources. All of the icons on the bottom right-hand corner of your screen (next to the clock) are using system resources (memory). Shut these down, if they're not needed. A lot of applications are automatically started when Windows starts. To stop this go to 'Start' and 'Run' and then type 'msconfig'. Again, depending which OS you are using under one of the tabs will be a list of programmes, which start with the OS. Untick any unnecessary ones, remembering to leave your Anti-Virus software programme checked. This should free up some memory.
The only other ways to improve/upgrade your system resources are:
1. Use a faster processor
2. Install more memory
3. Points 1 and 2
HTH