If you are using Windows NT/2000/XP, monitoring free memory is very easy. Simply right click on the task bar and open the Task Manager. Click on the Performance tab, and you'll be able to monitor the physical memory total and available on your PC.
If you are using Windows 9X, you will have to add some optional resources. Open Control Panel, then select Add/Remove Programs. Click on the Windows Setup Tab, and select System Tools, then click the Details... button. Check the boxes to install System Monitor and System Resource Meter. Click OK, and you will be prompted for your Windows CD. Insert the CD and Windows will add the monitoring tools. You can then use them to watch your free resources.