About modem sharing

I’d like to know’bout modem sharing only with learning purposes. Looking for “sharing modem” i’d got no issue in Help&Support centre, though I asked for help.
If you have not got tired from me here is one more question:
I can’t upgrade from Win98se(ru 4.10.2222 a)to Windows XP(ru build 2600.xpclnt_qfe.010827-1803) setup suggest only clean install.
* All hardware is mentioned in HCL
* All system requerements are met
* Windows2000 setup wizard suggests both types (upgrade & clean install)
*Clean install works perfectly on the same machine(Windows XP)