PC is very slow

Today I started my pc and it was very slow. Normaly it takes less than 5 minutes to start up and run all startup programs, today it took over 30 minutes.
I have a good pc and I really don't know what the problem is. I got good cooling and my pc is cool inside. I checked all the cables in my pc and they are all connected. I updated my videocard drivers to the latest version. I use avast as anti-virus so I don't think that I got any viruses. This also happened 2 months ago, I tought my gpu was broken so i bought it to a pc-engineer and he tested my gpu and it worked perfectly. While my gpu was gone I didn't use my pc for a week and when I got my gpu back, it all worked fine again. Please help me.
SPECS:
Windows 7 Ultimate Edition (Build 7600)
CPU : Intel Core2 Quad CPU Q8300 @ 2.50GHz
GPU : AMD Radeon HD 6900 Series
DirectX Version : 11.0
RAM : 4096 MB DDR2 Dual Channel

Check if your drivers are up to date, check if your programs and utilities etc. are up to date (Free Computer Security - Personal Software Inspector (PSI) - Secunia). It might just be that it's all cluttered up, the PC ain't that bad but then again it's not that new either.

You should really get new RAM, and for that, you need a new motherboard that supports DDR3.
And probably a new processor.
Get AMD if money is an issue.