Depending on how your network is configured and what hardware is used (particularly your router and any "smart" network devices), you can dedicate bandwidth to specific users on a network. This is a job for your network administrator as this appears to be your workplace.
If you want to make your work network better, have the system administrator take a look at it. If you don't have one, hire a consultant for a few days to sort things out and let you know what should be done and how much it will cost.
Software will do nothing in your case, especially if you can only install it on your computer.
Everyone in the office share the same network, and some are downloading. When I come to use the network, my internet speed is awful slow.
I want to know is there any software to ensure I have a stable internet speed?
So when I come to use the network, the software spare me a stable network.
If there is connection problem in your network the speed may be varied.
check your PC whether it have any malware problem.
clear the cookies and browser history's of your browsers.
Use Firefox,chrome or safari to increase the internet speed.
If your using a sharing network the speed of your net may be slow
Yes there are certain sites where you can check the speed of your network. the site given below is one of them scanmyspeed.com .you don't even required flash or java script to try on website. you can download it as a gadget or widget for your PC