You can get low level diagnostic software written in machine code. You boot from the floppy and run a series of burn-in tests which thoroughly test the hardware outside of the windows environment. If you cycle the memory enough times eventually intermittent errors will show up in the tests. Likewise the hard disk will show any errors. (
http://www.uxd.com/qtpro.shtml).
BUT memory faults are bloody difficult to diagnose. My gut feeling is that if you system has been running ok un touched, then the memory is probably ok. What exactly did the DOS hard disk failure warning actually say? It could be the disk is on the way out.........
Before you go ahead and fork out money on new hardware, I'd stick the windows disk in, boot from the CD and re-install Windows. When it asks which partition choose your existing C drive, and format as a normal format. Any errors on the drive will show up, and all data will be removed. Once windows has installed, if your problems are gone, then its a Windows thing (ME is terrible for symptoms like yours).
The WD 80GB hard disks, 7200RPM, 8MB cache run about £45 in the UK and are excellent, I use them all the time.
Good luck!
