Hard Drive performing slower than expected

Hello I have a WDC WD500AAKX-001CA0 Hard Drive in my computer but it is performing slower than I expected. Sometimes when doing little multitasking the computer locks up for a few seconds or a program does not respond. Also last night I was watching a movie and suddenly the hard drive begun to write (dunno why) and the movie was lagging :(. So I think there is something wrong with the drive or a...