Windows drive
Never install programs/application/software in Windows drive. Doing this will slow up speed of your PC. Applications or software in windows drive(which is mostly "c" drive) take more Ram and slow up your PC. Always install new software in some other drive.
Clean cache
Regularly clean cache for optimizing speed of your PC. You can easily find some software out there for free. I am presently using CCleaner and it is one of best software available. I also recommend this software to you.
Less applications
Getting a 4 GB or 8GB RAM doesn't means to dump all with recycle. Use application which are mandatory to you. The more the applications, the more the system will be burdened.
Disk-cleanup
Disk-cleanup is a windows utility available in control panel. Run this to delete non permanent files.
Use antivirus
One of reasons for slow computer is virus. Virus makes its copies and then slows down your processor.
In order to shield against virus, use antivirus software, there are many software available like avast, avg, ava and blah blah! but i prefer Windows antivirus and i also recommend it to you. It is free software to use. It has also a benefit that it doesn't slow down your PC. Download it and regularly update it, it will give you perfect protection against virus.
Disk derangement It is also a utility provided free by Windows. It can also boost speed of your PC.
By following these steps you can boost up speed of your PC. But the most important thing i found useful is to install all software on drive other than Windows. I have got 8 GB Ram and even that my pc worked as it was 128 MB Ram. Then a friend advised me for installing application on some other drive. I did it and results were surprising now i have many programs and software including Maple, Illustrator, Office 2013, Autocad 2014, logo makers, news apps and many other services but still my PC looks as if it were empty.