PC games running slower then usual.?

My PC doesn't run games as fast as it used to anymore. It used to be able to play Gears Of War on medium settings at perfect speeds, but now it slows down. Even my Halo 2 is running slower. I used to run it maxed out at perfect speeds, but now it slows down. My computer is almost 4 years old now, and it uses an AGP motherboard (P4S800). I am running Windows XP Professional using a Pentium 4 with 2.4ghz, 1.5 gigs of ddr ram, ATI X1650 PRO 512mb AGP, and a 550 watt power supply. Is my hardware getting to old?