Help my computer is running slow

Hi, please help me, my computer gets slow.. :( really slow..

these problem occurs a while ago when i shutdown my computer and plug in the cd rom into my computer, because i want to install something, by the way, the cd rom comes from another computer. then after i restart my computer, it makes my pc very slow.. the sounds become slow also when i am playing music, as i see, this occurs when the red led in my pc case lights up, meaning it is reading right? but before, even if i run games while playing music at the same time, the sounds are ok and the performance of the pc is also ok. how can i fix this? help me :( btw my computer is pentium D, 3.0mgz 512mb ram and running windows xp