rebuilt pc loads of data missing on hard drive?

[MofoKilla]

as above i recently upgraded my computer new mobo , cpu , ram etc put my storage drive in after windows was installed and there is loads of files missing maybe like 2gig or so.

can these files be recovered at all ?

also what would cause them to vanish ?

thanks,

 

[MofoKilla]

two complete folders are missing with about 2gig of files.

is there any programs i can use to recover the files?