Company names and domain names are not necessarily trademarks, though they can be.
Just because a trademark is not registered doesn't mean it's up for grabs. Trademark rights arise from the mark being used in commerce, registration just gives some additional protections.
Buying and creating domains doesn't give you any rights as to similar domains. Domains are just names, they have no inherent rights. Indeed, if you have been found to have obtained a domain in bad faith (to interfere with a company with that name or trademark), you an indeed be found to be cybersquatting and have the name taken from YOU in a UDRP proceeding.
Now rather than me writing an entire treatise on trademarks and domain names, why don't you give us some actual details so we can determine if their are trademarks in existance, whether there is likely infringement between those marks, and what the domain possibilities are.
For example, I have a company called Sensor Systems. I've owned the domain sensor.com since 1993. There are no fewer than four other companies called Sensor Systems out there with various perturbations on the domain name. We assert Sensor Systems as an (unregistered mark) though we've owned other registered marks for some of our key products. We've not had any infringement issues. Occassionally, I'll get a misdirected communication looking for one of the others and I just forward that along (or return it to the sender with the correct address). Our markets are very, very, fringely related (we do geospatial software and they make GPS antennas). We had a small lawyer-letter-writing discussion on one of our other trademarks but we settled with the fact that the products were really in different markets and we agreed to cross-recommend each other's product if we encountered one looking for the other (One was an satellite image analysis package called Xcalibur and the other a document imaging system called Excalibur).
