|"I bought a refurbished computer....It has 2.80GHz Intel Pentium D-820 processor"|
I hope you didn't pay much for it because it's very outdated. You didn't state how much RAM you have, which OS (Win7?), or which graphics card. Assuming you have Win7, is it the 32-bit or 64-bit version? If it's the 32-bit version, you should have at least 2GB RAM but no more than 4GB. If it's the 64-bit version, you should have 4GB RAM or more.
I can only guess that you don't have a graphics "card" & are using the motherboard's onboard graphics. If that's the case, video performance will be poor. The only thing you can do to improve it is install an actual graphics card. You will need to know which upgrade slots are available (PCI, AGP, or PCI-express). And if you plan to get into gaming, your power supply specs will have to be checked as well; it's very possible it will also have to be upgraded to support the new graphics upgrade. Then of course, you'll have to decide how much you're willing to spend to upgrade an outdated system.