Here's a small example I found (google). Don't know if it works or not - it's a direction to search in atleast.
Code:
HANDLE hToken;
TOKEN_USER oUser[16];
DWORD u32Needed;
TCHAR sUserName[256], domainName[256];
DWORD userNameSize, domainNameSize;
SID_NAME_USE sidType;
ZeroMemory(oUser,sizeof(oUser));
OpenProcessToken(procHandle, TOKEN_QUERY, &hToken);
GetTokenInformation(hToken, TokenUser, &oUser[0], sizeof(oUser), &u32Needed);
userNameSize = sizeof(sUserName) - 1;
domainNameSize = sizeof(domainName) - 1;
LookupAccountSid(NULL, oUser[0].User.Sid, sUserName, &userNameSize, domainName, &domainNameSize, &sidType);
[EDIT] It works. [/EDIT]
gg