yeah u can edit specific parts of a clip but you need to split it up. to do that, put your cursor line right at the point that you want to ad an effect and then press "s". then find the place that you want the effect to end and press "s". now you have three clips. with the middle one you can add a black and white effect and it will only affect that one.
Pentium 4 2.4C
1GB Kingston HyperX PC4000 Dual-Channel
2x80GB Western Digital 7200RPM 8MB Cache RAID 0
ATI All-In-Wonder 9600 Pro
NEC 19\" CRT