"is it possible to run the same program on two pc's in windows xp or does registration stop this?"
Legally it depends on the terms under which you have bought the product, For instance you are allowed to have a copy of MS Office on both a desktop and a laptop at the same time. Other programs will insist you buy a copy for each system. XP registration has nothing to do with this.
Technically it varies from program to program
"can a program be accessed via a home network?"
I presume you mean can you have a program installed on one system and run it from the other - not normally. Most programs have to be installed on the system you are running them on, otherwise Windows knows nothing about them. Some times they can be - depends on how the program is written.
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