"Usted piensa que yo la quiero." translates into "You think that I love her." where does the "her" come from? My translation "you think i want that" Does any one get this?
If you're talking to your girlfriend (I don't know why you would use "usted" for her, but...) it could translate to "You think that I love you." The "la" in the sentence gives the "her". It is a direct object pronoun. For your girlfriend (or a girl saying this to a boyfriend) you would probably say "Tú piensas que yo te quiero." Your translation "you think that I want that" is also correct. The intended meaning can only be determined by context or by asking the original speaker.
Like Freddyboy said, it depends on context. In "la quiero", "la" is the direct object (feminine) pronoun. It could mean 'you think that I love you" if the speaker were speaking TO a woman (although as noted above, its a bit formal). It could mean "You think that I love (or want) her, if the speaker was speaking to someone else about another girl or woman. I could also mean You think I want IT (referring to a feminine object). I think "you think I want that" would translate to something like "Usted piensa que yo quiero ese/a/o" (although I'm not sure- The various endings of este and ese still confuse me)