I have a dimension 5150 encountering same problem failure to connect to internet through wireless router. This router works with two other computers. Worked with 5150 before clean reinstall of os.
Have consulted 4 different forums, to no avail.
Running win XP Home Edition, 3.2GHz, 1GB RAM, 160 GB HD.
Have clean reinstalled os, loaded drivers in correct order, installed NIC drivers from 3 different sources (Intel, dell, MS). Still get 'software not installed for Intel pro/100 ve network connection' in device manager. Last forum has given up and suggested here. 
Also get message 'network connection is missing', but have seen this before with functional internet connection.
Inquiry to network adapter indicates device is working properly. Device manager says software is missing; also indicates Multimedia Video Controller is not installed (?). LAN is listed under network connections, but no wireless. 
I am a mid-level computer person. It seems obvious that the network drivers will not install on this system, but nobody can tell me why.
I think after all this that I must junk this hd and install a refurbished one; cannot afford new one. I was attempting refurbishment of this desktop at minimum cost. Apparently this is not possible.
If you have any additional information, I would appreciate any solutions.