Could you provide me with a step by step guide to cleaning a PC hard
drive completely and re-installing Windows 98?

As mentioned in a comment, check out this site:
http://www.dansdata.com/sbs30.htm

It talks about backing up your data, copying the WIN98 directory from
the CD to disk, etc.  If you are just wanting to completely scrub the
contents on your hard drive and THEN install Windows 98, you can do
the following:

1.) Get your hands on a Windows 98 start disk (a floppy).  If you
currently have Windows 98 installed you can make one by following the
steps at:
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q187632

Follow the steps in Method 1 to make the Startup Disk have the FAT32
files needed.  (FAT32 is a file system that windows can use that
allows for larger storage devices; if you use FAT16 (the default) your
have to partition your harddrive up into 2 GB chunks, which can be
annoying.  See http://support.microsoft.com/default.aspx?scid=kb;en-us;Q154997
for more info.)

2.) Pop the Windows 98 Startup Disk into your computer and restart. 
The computer will reboot and load off the floppy - it should prompt
you if you want to enable CD support - choose that option.

3.) Once you get to the command line, enter: fdisk
This will totally scrub your hard drive.)  It will ask if you want to
use the FAT32 support - say yes.

4.) In the fdisk menu first choose (1) Create DOS Partition or Logical
DOS Drive; then chose (1) Create Primary DOS Partition.  It will then
ask you if you want to make it the largest size - say yes.

5.) You are done with fdisk after this.  Back all the way out back to
the command line and enter: format c:
This will format your hard drive, destroying all data.

6.) When you've finished formatting, you're ready to reinstall Windows
98.  Your harddrive is 100% clean now, I promise!  :-)  Reboot your
computer, loading off the Windows 98 Floppy again.  Opt for CD-ROM
support (again).  When you get to the command line, navigate to the CD
ROM drive (probably D:) by doing: cd d:\setup  (replace d: with e: or
whatever, if your CDROM is a different drive letter).

7.) Type: setup
You should be off and running on a fresh new install of Windows 98! 
Congratulations!  :-)

You can read a very detailed account of the instructions I presented
at:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q221829

Happy computing!

Hmm ... 
://www.google.com/search?q=Step+by+step+guide+to+reinstalling+windows+98
gives as the top two sites ...

(1) Step By Step - Reinstalling Windows
... support our sponsor. I regularly get e ... Windows 95 and 98 will,
in normal use ... Step three. ... data files. After reinstalling,
you'll have ... in your WIndows directory, by ...
http://www.dansdata.com/sbs30.htm - 23k

(2) winstall.com site index for installs reinstalls and ... 
... Click here to make ... one. Mark?s Guide to FDISK Microsoft ...
Install Step by Step. ... Privacy Policy. Reinstalling Windows Remove
... up for Windows 98 Scandisk/Defrag ...
http://www.windowsreinstall.com/siteindex.htm - 35k

regards, sabre23t =^.^=

Hmm ... GA doesn't allow me to edit my comment. So comment on comment
then ...

(1) That http://www.dansdata.com/sbs30.htm is really good,
reinstalling Win95/98 in seven easy steps. It confirms all the hard
learned stuff that I had gone through about reinstalling Win98, and
also a few that I didn't think of.

(2) That http://www.windowsreinstall.com/siteindex.htm links to
http://www.windowsreinstall.com/install/waysofreinstallingwindows.htm
. Give more ways of reinstalling Windows, and main site have loads of
other reinstall related resources, but I believe dansdata site give
better advice for reinstalling Win98.

regards, sabre23t =^.^=