BT is 100% accurate, I guess I did not explain it completely in my last post. When your PC lacks adequate RAM, it cannot process information at an optimum level. RAM works VERY fast, in terms of Nanoseconds, so when your CPU needs data, and that data is brought from your Hard-Drive into RAM, it can be accessed fairly quickly. The CPU may be the fastest made, but if it can't get the data it needs to it quickly...well your CPU is SOL, until the data arrives.
If you don't have enough system RAM, this data the CPU needs is instead 'stored' in what is called "virtual memory". This is a "space" allocated on your HARD-DRIVE to hold the data the CPU needs until it can be moved into RAM, and then the CPU's cache as needed. So...essentially the virtual memory, or "page file" in your Windows OS is acting as RAM, but it works somewhere in the area of milliseconds, instead of nanoseconds...which is an eternity in terms of a speed difference in computer terms. Hard-Drives ( where the OS page file is located at ) are extremely SLOW in data throughput, compared to RAM. Hence, LAG, or even a system lockup, as if your PC runs completely out of RAM and Pagefile, your OS can find itself... with no room left for itself to operate in and your PC can crash or lock up.
Some games can do this, and you'll hear about things like "memory leaks", which is basically a way of saying over time, the program is leaving no room left for the OS and you get a CTD ( crash to desktop ) or some similar type of thing.
Most bargain brand, or off the store "shelf" PC's are very LIGHT in system ram, and "many" are not really designed with gaming in mind. A good rule of thumb, is 2 Gigs of system ram for XP and even MORE for Vista, for things to run really well when gaming. The more intense your gaming...the more RAM you need !! Same rule applies to any Vid-Cards...generally the more RAM on the PCB the better off you will be. I know with your situation and a laptop...Vid-Cards don't really apply...just mentioning it.
LOL...I can't program or model a single thing, but make PC's I can do, and have been for over a decade now...sure beats buying store bought stuff