If you have a windows98 startup disk boot it up and copy himem.sys from the floppy to the hard drive.

here's the syntax in case you don't know how:
(this is all at the 'a:\>' prompt)
'a:\>copy himem.sys c:\himem.sys'

If you don't have a Windows 98 startup disk, you can create one on any windows 98 computer using the windows 98 CD-ROM. Click start, then settings, then control panel. In the Control Panel, double-click on Add/Remove Programs and then click on the Startup Disk tab. Insert a blank floppy and the Win98 CD in their respective drives and click the Create Disk button. After this is complete, follow the steps above to restore himem.sys.