This is something that a lot of people were finding difficult to address.
Now bid good-bye to all your worries 
, well!!
I’ll cover sharing files step by step accompanied with screenshots so that its easy to understand.
Step1:
On your Solaris host, before running the guest(XP in this case), choose Settings->Shared folders->Add folder. (First create a folder in solaris intended to be shared with xp then just navigate to it as mentioned above).Then boot your ‘Guest’ 
Step 2:
Start menu ->(rt click) ->My Network places->Entire network ->Virtualbox shared folders
Here if you have created a Share folder as mentioned in ’step1′ then you have to see an entry under “Virtual Shared folder” with an appropriate name.(You have to see it, otherwise check your unix commands )
Step 3:
(rt click) on that shared folder -> Map network drive.
Keep the default name(Z:\ in my case) or if your really exited that your almost approaching a successful end then go ahead and change it .
Step 4: Thats it!! 
Now when you open up My computer, you will see the newly created drive under “Network drives” . It opens up like any other drive and operations thus follow in the usual manner.
At the end this is how it looks!!!
Cheers!!
