This is essentialy an add for a comercial package, but is on topic so have moved it here.
Recommend good calculator and grapher to you. Just visit Graph Functions, Graph Equations, Create Data Graphs.
To CaptainBlack: Sir, I can only say sorry and thanks.
To ^_^Engineer_Adam^_^: Math Studio for school teachers and students; Function Grapher for college and university teachers and students, it's a good tool for calculus; and Visual Data for researchers. Thank you.
Some quick changes to your system can be preformed in the registry settings, just be careful what you do, it can kill the OS. Many times you can change expiration dates from there. You can also try reseting the system clock, but that is a very primitive way and usually never works. And if you really need it badly you can reinstall the Operating system.