As mentioned above it's your CPU that's doing the hard work for rendering.

Appart from a low specification CPU, slow rendering could be one of many things:

My favourite switches to slow down rendering are:

Caustics: A 2s/frame render of a transparent flashing light for an ambulance slowed down to 2 minutes
Raytraced Depth of fields: 1s/frame changed to 3minutes.

The codec you export to may also cause problems. I normally render to uncompress AVI then use a conversion tool to change it to WMV.

Finally if your whole image can't fit in ram then the disk will start swapping and this could result in a 20-30 times slow down. Check task manager's performance tab, Commit Charge Total vs Physical Memory Total (or the equivalent if you are on a Mac)