Actually, topic says "put computer right next to monitor" but your question involves putting your sub close to your monitor.
Anyway, no, there should be zero problem with putting your monitor and computer next to each other. As for your sub, that probably depends on what type it is and how close you want them to be. If it was designed for use with a computer then you probably won't have any problems at almost any distance. If it wasn't then I would suggest you keep at least 18" between the sub and the monitor (distance from floor to desktop should be suffecient).