HI I have dell desktop tower computer. what I want to know if can place the cpu box flat instead of standing.

Yes you can place the CPU Box on its side as long as you do not cover any vent openings effecting the air circulation. Here is the technical side of your question; You will not affect any electronics of the computer, but we use to recommend reformatting the hard drive on its side because the hard drive head rides above the disk surface micro meters from the platter. The gravitational force on the head may affect the positioning of the head over the platter surface affecting the reading and writing of the data. However, the technology is so much better now that I would back up any critical data before you lay the box down on its side. Next I would right click on your drive and left click on tools to defragmenter your drive.