Few more details?
Its a bit difficult to diagnose a machine without knowing any of the details! If you could explain what software its running, what hardware specs it has, and other specific symptoms, you've got more chance of getting a useful answer.
One thing I would suggest is investigating how much software is installed and running in the background from startup. Not too sure about Asus, but lots of manufacturers insist on bundling endless software packages with the pre-installed OS. Most of this software is useless, or just not suitable for your needs, but it sits there and consumes system resources regardless.
I normally format any new computer that has preinstalled software, and install my OS of choice, with only the software that will actually be used. Also, it's wise to keep track of whats being installed day-to-day, and whats enabled at startup. regular clean ups of unused software, along with registry cleaners will help keep the system clean and responsive.
Finally, watch out for installer programs that sneak in 3rd party software trials when installing a legitimate application. Blindly clicking next next next is almost certain to end up in lots of unwanted toolbars, "optomisers" and other junk.