#1 Thing to do is get rid of the 100gig drive I had a friend just buy a new laptop with 640Gig a video card and 4 gigs of ram duel core processer for under $500 USD
#2 delete as much "junk" you don't need( Music you can always get again) pictures files folders programs ...You could always upload your pictures on FB or photobucket you can save your music on a MP3 player or Burn it on a CD Memory stick
Once you deleted Moved your "junk" download Ccleaner or even better (Advance system care) both free and no virses
And yes defrag your harddrive
Having a full hard drive is what slows you down and ya LAG
I run a 10,000 RPM 600Gig 32mb cache WD VelociRaptor Sata drive. And a 2 TB WD Caviar Black for Programs Video < hehehe and music
Games : Wolfeinstien, IL-2 Stormivik Cliffs of dover, Silent hunter 4, NavyField, Battlefield free4play
Programs: FacetrackerNoir,Webcam software, ATI manger (videocard) slimdrivers, Advance systemcare 4, Logitech HD, Fraps, Spaccy, Ccleaner, MSI afterburnner ( Overclocking videocard) Joystick driver + more and all my window 7 apps Arobe Java ETC
I still have only used 7% of 100% in HD 2 and 11% of 100% in HD 1 so my 11% would be 110Gigs
Depending on what your computer is Look into getting Ram aswell ...Running less then 4 gigs now adays is like running a 100gig harddrive ..thing of the past