I went to the site and searched for PCL841. PCL841 is a PC interface card which communicates with CAN controllers, it has nothing to do with RS-232. you can use this card to communicate with microcontrollers that have CAN controller. are you a windows programmer I mean do you have Visual Basic programming skills ?
if you want to read temprature every 5 minutes you can happily use RS-232 which is much simpler and cheaper.if you want to use RS-232 you don't need PCL841. having said that, noise level of your environment is also important. how much noise do you have there? and what is the distance between computer and sensor ?