It could be a myriad of possibilities.

Have you updated your graphics driver or changed any hardware since a year ago? Windows may have also automatically pushed a driver update that just might be causing the bugs.

Are you running in RAID-0? Perhaps your physical hard disc is having issues. Do you have multiple hard drives and could attempt to install it on a different one to see if this makes any difference?

Are you installing STW: Warlord's Edition from the same medium as originally used a year ago? (i.e. are you installing it from the same CD that you originally bought?) If not, perhaps it's at a different patch level that may not agree with your system?

I can give other scenarios, but I'll stop there for now.