PC suddenly slow? CPU won't run at full speed?

I have just solved an issue on my Dell inspiron 1520 (C2D T7300 Vista) which had me going for a couple of weeks. I was about to ask for advice on this forum but now find myself in a position to offer some instead. Reading discussions on Speed Step allowed me to at least find a workaround, keeping the laptop useable until I eventually found out what was going on.

Suddenly my laptop went super slow, talking 15 minutes to boot. Thinking it was the software I had just loaded I uninstalled it. No better, so I started blaming Vista... as you do. I had not yet set a restore point (tsk tsk) and did not fancy a rebuild.

If I looked at the Vista Resource Manager (in task manager), the CPU was flat lined on 39% Maximum Frequency (blue line). I then thought it was something to do with the power plan mucking up. Changing this or creating my own made no difference to the CPU frequency. Occasionally after cold booting on battery it would come good for 20 mins or so. I went through the processes of Virus and Spyware checks then came across the discussions on Speed Step. I went into my BIOS and disabled this feature, suddenly my laptop was working better. I again started blaming Vista thinking it had lost control over this feature. Soon I started thinking, geez this laptop's been running fairly quiet of late, I wonder why the CPU fan is so quiet.

Wanting to confirm if the CPU fan was kicking in I started looking for utils that would give me the CPU temp (I8kfanGUI is a good one). CPU was reaching 70 deg quite often and once made it to 80 so I thought i'd better get onto Dell. They asked me to put in the drivers disk and boot off it so as to run their diagnostics. Sure enough the diagnostics failed with the CPU fan. Once replaced I turned Speed Step back on and the laptop is good again.

I learnt a couple of interesting points with all this.

1/ The Vista operating system, Dell BIOS and features do not notify you if the CPU fan has stopped working. I would have expected an error to pop up somewhere.

2/The computer was smart enough to scale back its CPU clock speed to a constant 39% Maximum Frequency once it sensed the CPU fan was broken. Disabling Speed Step improved this and bought me some time to investigate the problem at the risk of running the CPU too hot.

3/ When Speed Step was disabled, the Resource Monitor showed the CPU running 100% Maximum Frequency. Vista, I8kfanGUI and Notebook Hardware Control all reported the CPU to be running at 2000Mhz. I though believe CPU-Z which reported the CPU at 1200Mhz using a multiplier of 6. This is because the laptop still seemed to be running a little slow. Once the fan had been replaced, the multiplier as seen with CPU-Z happily danced between 6 and 10.

Doing the maths here suggests the following: with the CPU fan not working and Speed Step enabled, the internal clock speed had dropped from 200Mhz to 130Mhz and the multiplier from 10 to 6 as (0.39 x 2000)/6 = 130. With Speed Step disabled, the internal clock remained at 200Mhz but the multiplier was reduced to a constant 6.

I hope this gives another avenue of thought to those researching why their computer is suddenly running slow. It might in fact be the CPU fan which has stopped working.

Yeah thanks yay, I was also amazed that neither Vista or Dell have a utility for checking CPU temperature or to show you the fan RPM's built in!