three things
first hold down Ctrl+Alt+Del then select task manager, then select performance, if the cpu usage history is constantly above 75% close some programs by going to the tab processes and look for any processes that have a high number, make sure it's nothing important, generally if it has the word "windows" in the description don't close that program, close a program by right-clicking and selecting end process, if the performance is till constantly above 75% then your cpu is likely not up to the job, so then do one of two things, buy a new computer with a faster processor or uninstall programs until you see the performance go down, then reinstall the programs that didn't drastically change the performance, uninstall a program by going to start, computer and then in the top you'll see uninstall or change a program, select the program then select uninstall, follow the steps
secondly it might be the memory, same thing, Ctrl+Alt+Del, go to performance, see your physical memory usage history, if that is near the top then install some more memory, in most all computers you can do this by removing screws in the bottom and install memory you just bought, you can buy memory from many places, start with amazon, http://www.amazon.com/s/ref=amb_link_6500672_25?ie=UTF8&rs=1292120011&bbn=1292108011&rh=n:541966,n:1292108011,n:1292120011,p_6:ATVPDKIKX0DER&pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-4&pf_rd_r=0XSY9JDWBPEBV9PJ9GDD&pf_rd_t=101&pf_rd_p=93651062&pf_rd_i=193870011
newegg.com is also very good for value
depending on your computer you might have different speeds of memory 533mhz or 667 is likely, you might have 800mhz depending on how much your computer cost originally, check the specs of the computer by typing in the model number online, most likely it will be 667mhz, also their are a certain amount of slots on all computers, most likely you'll have 2, check before you buy by looking in the computer, also check the total memory by going to the system properties, if you have 2 512mb then it will say 1GB, if you have 2 1024mb then it'll say 2GB, if you have 1 1024mb then it will say 1GB, simple math, make sure more memory then your computer already has, another tip is if you have two slots and one 1024mb card, buy just one more, not two
thirdly, your hard drive, start then computer, what your looking for is free space, if it's in the mb range for OS(C:) then your hard drive is probably getting to full, delete some files, larger ones preferably, two programs that will help you with that is WinDirStat, but make sure you don't delete anything important, again stay away from the the windows folder, a don't delete a file that is exactly the same size as your memory, it's important
lastly before you do any of this make a backup of your hard drive, you should always do this before monkeying with your hard drive, http://download.cnet.com/SyncBack-Freeware/3000-2242_4-10413802.html?tag=mncol , search around backup programs, they are very different, some simple, some complex, you'll need an external hard drive to back up on, if you don't have one buy one
http://www.amazon.com/External-Hard-Drives-Storage-Add-Ons/b?ie=UTF8&node=595048
I'd recommend seagate and get something big, 1TB or more, because you'll likely make backups of future computers that will likely need more space, and it's a pain to buy a new hard drive every time you need to backup a new computer
oh and in the future understand that buying a cheaper computer is fine, just understand it will lead to problems like this sooner, which can easily be resolved as long as your geeky or just experienced with computers, which should be a synonym, to make sure problems like this don't arise in the future monitor your computer every so often to see how full the hard drive is, how much your memory (ram) is being used, same with the cpu, lastly empty your recycle bin every so often
http://download.cnet.com/WinDirStat/3000-2248_4-10614593.html