I often get people asking me to come over and fix their computer because "it's running really slow". I find that the majority of the time, it is simply one (or both) of the following 2 things that are causing their computer lag.
1 - Clear up that hard drive!
Windows needs space on your hard drive to work well. It's always moving things around, loading files in and out, creating temporary files to work with etc. In order to do this, it needs free space on your hard drive. Take that huge mp3 collection (bought and paid for from iTunes, of course!), large photo folders, video folders, and anything else that tends to take up lots of space and move them elsewhere. Check under My Computer. The "C" drive will be your main hard drive. Many times, your computer will come with a partition already set up. If there are any other drives listed under the "Hard Drive" section, then it is likely a partition (or even a completely separate hard drive). Simply click and drag the folders containing all your large storage folders over to your second drive. If you don't have a partitioned drive (or a second hard drive), you can burn your files to a CD/DVD, or there are even online storage sites that will keep your stuff "in the cloud" for you. However, most of the sites don't offer a great deal of space for free. Up to 2 gig seems to be about the standard. External usb hard drives are another option. They have gotten a lot cheaper in the last year. It should be fairly easy to find a 1 terabyte (1000 gigabytes) of storage for under $100. Future Shop, Staples, Best Buy or your other favorite tech store will have one. However you choose to store your files, you should be aiming to keep at least 20% of your "C" drive free.
2 - Clean out that spam and spyware!
No matter how "careful" you think your internet habits are, you are going to pick up spam bots and spyware. The two programs I choose to use currently, both of which are free, are SuperAntiSpyware and MalwareBytes. SuperAntiSpyware runs in the background. MalwareBytes does not, and needs to be run manually when you want to do a sweep. Keep both up to date, run scans with both of them once a week, and your computer will stay clean and speedy.
As simple as it sounds, these two steps make up a good portion of the calls I get for slow and laggy computers. Keep on top of these two things, and your computer will likely stay quick and speedy. At least, until its a month old, in which case it will be horribly out of date and due to be replaced! The joys of the technology age...