:D Found the solution atlast!
It was the DMA setting for the hard-disk..I forgot to re-enable it after installing 98(The DMA setting will be turned-off by default in win 98 and it will be on by default in XP..thats the reason why I havent faced the problem in XP).Now the video playback smooth again and no more jery frame-rates and sounds.
I found the solution while browsing through the UBI's POP SOT Forums..someone had the same problem as I had..
I hope this helps others who might have a similar problem.

PS: I've replaced all the missing .vxd files(went through all the devices in the Device Manager)..but I dont see any visible difference in performance..maybe I'll have fewer crashes..anyway it was something that I never knew before..Thanks for the info :)

Anyway thanks a lot guys..TweakTown Rulez!:cheers: