Need help restoring valis IP address..... system has given itself an "169" number which is causing "limited to no connectivity"

All worked fine until I tried to install a wireless router.  Actrually even that worked fine until I shutdown the desktop because it had froze up(system is a dimension 2300 and about 5 years old).... after that the internet showed little to no connectivity.  I called Comcast which tried to fix it and after many attempts told me that the system gave itself an IP address that was causing the problem.  They told me that Dell would have to clear it, and reset the old one which would start with anything like 24, 76 etc etc.... they also said that dell might need to 'remove the network card'.....  Since the warranty is up Dell is charging at least $50 to listen to the problem and since I am planning on buying a new desktop within the month I don't want to pay.  Is there any advise on this, so that I can try and fix it myself?! Thank you in advance.