It never ceases to amaze me how people jump to the conclusion that Solaris has a problem. You have a Windoze system involved here and that's the first place to look. What you are describing would be a major defect in a Sun/Oracle flagship product. I can tell you from experience that if you tell Oracle that you've found a major defect in a flagship product, they'll ask you to prove it, and if you prove it, all hell breaks loose. It gets fixed pdq. No such thing with Windoze. You'll probably get no reaction from MS or they'll even claim it's a "feature".
Network performance in Windoze is a known problem (especially if you are using gigabit interfaces) which has caused masses of bust-ups. Read this link just as an example, there's plenty more.
Are You Seeing Slow Windows 7 Network Performance?
It just goes on and on.
I recommend moderators move this thread to the Windows forum.