Disk access issues

[Melthalas]

My pc's been giving me trouble since the last 2 or 3 days. It freezes for 5 to 10 minutes then starts functioning as usual but slower. I have noticed thanks to the task manager that the problem is that the disk access goes up to 100% so the programs have to stop responding. The thing is that when that happens, task manager doesn't show any process doing disk operations, or at least not ones that would take it to the 100%. My question is that if there are any processes that the task manager doesnt show or something of the sorts, so I can find the source of the problem and with any luck solve it.
I am almost certain that I have been infected with some trojan or worm that is "clogging" the disk access so I have run several scans with my antivirus (Bitdefender) but with no results. I am (unfortunately) running windows 8.

I hope you guys can help me. Thanks in advance

P.S. : Sorry admins, I didnt know where to open this thread so feel free to move it appropiately.

[Shay]

Run Malwarebytes Anti-Malware
http://www.bleepingcomputer.com/down...-anti-malware/

Double-click mbam-setup-2.0.0.1000.exe and follow the prompts to install the program.
At the end, be sure a checkmark is placed next to the following:
Launch Malwarebytes Anti-Malware
A 14 day trial of the Premium features is pre-selected. You may deselect this if you wish, and it will not diminish the scanning and removal capabilities of the program.
Click Finish.
On the Dashboard, click the 'Update Now >>' link
After the update completes, click the 'Scan Now >>' button.
Or, on the Dashboard, click the Scan Now >> button.
If an update is available, click the Update Now button.
A Threat Scan will begin.
When the scan is complete, if there have been detections, click Apply Actions to allow MBAM to clean what was detected.
In most cases, a restart will be required.
Wait for the prompt to restart the computer to appear, then click on Yes.