That sounds like a dying drive to me.
Hard Drives can die in a variety of ways, often they just fail completely, but sometimes you will get this situation - where the drive will have trouble being recognised and will behave incredibly slowly. The length of time in this state is unpredictable and depends on a massive array of factors, but needless to say each time the drive is power-cycled its remaining life is slashed.
My advise would be to salvage as much as possible from the drive, identify important things such as Documents/Pictures first and ignore the larger items such as programs as they can be downloaded again.
Another option if you are willing to risk the time, is to try and take an image of the entire drive - this can then be imaged back onto a new drive. This method will preserve the Windows installation and configuration, whereas the above will require a reinstall.
If you wish to do a bit of diagnostics first, you could lookup the S.M.A.R.T information for the drive, using a tool such as Crystal Disk Info. Factors that would lead to a dying drive are:
- Reallocated Sectors Count > 0
- Current Pending Section Count > 0
- Uncorrectable Sector Count > 0
- Very high numbers of Power On Count/Hours (in the tens/hundreds of thousands)
On a personal note: This happened to me very recently, however the drive I lost was a member of a RAID-0 data array. It took several attempts to get Windows to recognise the array, after this I left the PC for several days removing data at ~2MB/s. I feel your pain!