When it comes to problems I cannot figure out, I use the three R's...Reboot
in MS DOS mode, Reformat, and Reinstall!  Takes me about 4 hours to get the
system back up and running like before (better, actually....like NEW!).  Plus
I heard that it's good to reformat your HDD about once a year anyway to keep
the system fresh.  After time, the registry can become bogged down with
obsolete entries, errors and other "stuff" which gets filed in the folder
called "Mysteries Unsolved"!  

A clean install just feels so nice!  Like a shower after seeing two grown
men grope each other in public.

1.  Boot to a DOS prompt and run SCANREG /RESTORE.
Select (if available) a date when this PC booted OK i.e.
before the trouble started.

2.  Boot into Safe Mode (usually by holding down the Ctl key
during reboot) and look at /Control Panel / System / Device
Manager where yellow triangles signal problems with
devices (which include keyboard and mouse.)

3.  Your intermittent problem of DOS prompt and lack of
keyboard suggests the problem may be on the motherboard.
(No keyboard driver should be needed for a DOS boot.)
We cannot forecast whether all you need is to vacuum the
interior of the PC clean or replace the PC or MB.

Because it ultimately does boot & work well, I have to think the Windows
installation is probably basically good, meaning all your boot files,
the MBR (Master Boot Record) & the PBR (Partition Boot Record) are
intact.

(1) Get the manufacturer's diagnostic program for your hard drive, to
see whether it picks up a problem. Here is a list that glee once
posted...

OnTrack Data Advisor:
http://www.ontrack.com/freesoftware/#dataadvisor
IBM/Hitachi Drive Fitness Test:
http://www.hgst.com/hdd/support/download.htm
Western Digital Data Lifeguard Tools:
http://support.wdc.com/download/
Quantum/Maxtor PowerMax:
http://www.seagate.com/ww/v/index.jsp?locale=en-US&name=PowerMax_4.23&vgnextoid=
a37d8b9c4a8ff010VgnVCM100000dd04090aRCRD
or
http://www.majorgeeks.com/Maxtor_Powermax_d1386.html
Seagate SeaTools for DOS:
http://www.seagate.com/www/en-us/support/downloads/seatools

(2) Close all open windows; "START button, Run, DxDiag".
     This is the DirectX Diagnostic Tool. Could it have a message?

(3) R-Clk My Computer, Properties, Performance tab.
     Are you "optimal" or "compatibility".

(4) R-Clk My Computer, Properties, Device Manager tab.
     Are there red/yellow ?'s/!'s on any icons? (If you end up in blank
     Windows Help Troubleshooting screens, post back for the cure.)

(5) "START, Shut Down, Restart in MS-DOS Mode", & enter
     "Scanreg  /Fix  /Opt"  (not the quotes).
     The Registry is involved with nearly everything. (Won't work for a
     Registry around 9 MB huge.)

(6) Do a Scandisk & a Defrag, but don't let them constantly restart.
     If they try, post back for a cure. Also...

     I must warn that Scandisk may not do a great job when there
     is a great one to do...
     http://cquirke.mvps.org/9x/scandisk.htm
    Therefore, perhaps UNcheck "Automatically fix errors" on it's front
    screen. It will then ask whether you want each error fixed. If it
    sounds truly horrible, say "No!", and post the log. The log will be
    "C:\Scandisk.log".

(7)
http://support.microsoft.com/?kbid=188867  Troubleshooting Startup
http://support.microsoft.com/?kbid=187602  Troubleshoot
http://support.microsoft.com/?kbid=192926  Clean Boot Troubleshoot

It sounds to me like you have an intermittent hardware fault, possibly
bad RAM or bad connections. I'd remove the RAM sticks and reseat them.
I'd do the same for your cards while you're there, and clean the dust
out of your fans. Alternatively, you may have a power supply issue, or
bad capacitors on the motherboard.

- Franc Zabkar