Windows 7: very slow startup and slow performance over all

its about 6 months i m using the same window, but now it starts really slow and over all performance of my system is also down.
i have dell n5110 i5 with 4gb ram,
and please i dnt want to reinstall window
also i am using selective startup



Any suggestions?

Try running the Dell diagnostic test, and download the diagnostic program from your hard drive's manufacturers web site. If it is Western Digital you can download WinDig here. You can also hold down the windows key ( the one with the flag on it) and press R key. In the box that comes up type msconfig then go to the startup tab and take as much as you can out. Mine has my anti virus and windows sidebar open at startup and nothing else. That does not mean they won't work, but only that they don't start at startup.

Edit: You should also take off all the programs and services Dell puts on your computer that you don't need or use.

honestly, the number one thing I have found is, when people reinstall windows, they see the ( your computer is ready to use) and start installing stuff and doing stuff.
There are at least 3 to 5 hours of updates. So, try to do your windows updates. There should be plenty. And keep running them untill it says there are no more then run them one more time. This normally fixes MOST issues of speed. and other than that, eronious progrmas running. example, if you install something from some program it will in 90% install add on software which you may not even want or use. ( alwaysy choose custom install and UNCHECK or check things but reading is fundmental.) clear those out. Deletong things DOES NOT MAKE OUR COMPUTER FASTER.