You are limited to 2 processors, I don't know which you have, see the specs:
http://shop.lenovo.com/ISS_Static/WW/AG/merchandising/US/PDFs/valueline_desktops_techspecs_062509.pdf Depending on your OS, Windows Vista, right? Is it 32 or 64 bit? If 32 you can add up to 4Gb's of Memory, it will only use 3.80GB's or so, with 64bit you can add more. Perhaps your Computer needs a Clean up. Her are some tips to speedup Windows Vista:
http://www.pcworld.com/article/143538/windows_vista.html OR, http://windows.microsoft.com/en-us/windows-vista/optimize-windows-vista-for-better-performance OR,
http://windows.about.com/od/customizingwindows/ss/speed_up_windows_vista_all.htm Try these freeware programs to speed up your machine and remove the junk. 1, CCLEANER at :
http://www.filehippo.com/download_ccleaner/ This freeware program has many useful tools the cleaner will remove junk and free up disk space. In the TOOLS section, STARTUP section, disable ALL unnecessary startup programs, most software you add want to start with windows and adds a software update to your machine which slows down your computer. Remove ALL unnecessary toolbars and Search providers from broswers they are killers on broswer speed.2, Download freeware MALWAREBYTES at:
http://www.filehippo.com/download_malwarebytes_anti_malware/ In the install UNCHECK the TRIAL PRO version, you want the freeware version. Run a FULL scan, delete ALL results, reboot. Run Windows Updates for current updates. Post results for further assistance.