You should be able to locate the logo.sys file (do a find/search) , it will be a .BMP file disguised as a .dll , MS likes to fool everyone.
Once located, open it with irfanview and it will tell you that it is
a bmp file and ask to rename it, say no and it will display it anyway. You should see the Win98 logo. Rename it to logo.bak, find any suitable image for replacement, size to 320X400, save it back to same location as logo.sys, that should do it. (Should be a .bmp or .jpg)
If you just wish to remove it, find the msdos.sys file in your c:\ dir right click and click on properties and remove the read only attribute. Open the file in notepad and replace the line Logo=1 by Logo=0
add line if necessary, but make it =0. Then save it back.