If you want to convert from std::wstring to const WCHAR* (i.e. the returned pointer gives read-only access to the string content), then calling std::wstring::c_str() method is just fine:
std::wstring wstrProcToSearch;
std::wcin >> wstrProcToSearch; // input std::wstring
// Convert to const WCHAR* (read-only access)
const WCHAR * wpszProcToSearch = wstrProcToSearch.c_str();
Instead, if you want to modify std::wstring's content, things are different. You can use &wstr[0](where wstr is a non-empty instance of std::wstring) to access the content of the std::wstring(starting from the address of its first characters, and noting that characters are stored contiguously in memory), but you must pay attention to not overrun string's pre-allocated memory.
In general, if you have a std::wstring of length L, you can access characters from index 0 to (L-1).
Overwriting the terminating ' ' (located at index L) is undefined behavior (in practice, it's OK on Visual C++, at least with VC9/VS2008 and VC10/VS2010).
If the string has not the proper size (i.e. it's not big enough for your needs), then you can call std::wstring::resize() to make room for new characters (i.e. resizing internal std::wstring's buffer), and then use &wstr[0] to read-write std::wstring's content.