Year Old Computer Has Slows Down When view flash or video

So my computer keeps slowing down whenever streaming video or when viewing any flash file. I've made sure that I have all the latest drivers, made sure everything was installed correctly including the video card., even cleaned the inside of the computer. I've tried this with both Vista and XP and the same thing happens both were at SP1 and SP3.

Here are my computer specs.

Intel Pentium D 935 Presler 3.2GHz LGA 775 95W Dual-Core Processor

SAPPHIRE 100195L Radeon X1650 512MB 128-bit GDDR2 PCI Express x16 Video Card

G.SKILL 2GB (4 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory

SAMSUNG SpinPoint T Series HD501LJ 500GB 7200 RPM SATA 3.0Gb/s Hard Drive

MSI P6N SLI-FI LGA 775 NVIDIA nForce 650i SLI ATX Intel Motherboard

Broadway Com Corp P4-OKIA600-BLACK 600W ATX Power Supply

KDS K-22MDWB Black 22" 5ms (2ms gray-to-gray) Widescreen LCD Monitor with HDCP support


When I first built this computer it was a beast, no it's more like a munchkin. I've tried reinstalling vista and xp many times but it doesn't get better.