RAM randomly goes up to 100% in windows 7

Hey there,
I recently switched to Windows 7 from Windows XP, and something I have noticed in when I'm working is, suddenly my RAM usage goes up to 100% , and my PC becomes damn slow.
Is there any chance that it may occur due to high RAM usage ?
These are my PC specs:
CPU: Quad Q6600 @ 3.2GHz | GPU: Radeon HD 6950 | Mobo: Asus P5Q P45 | RAM: OCZ 4GB 1066GHz | HDD: 2x 1TB Samsung, 1x2TB
Thank you.
