diff --git a/docs/rh-log.txt b/docs/rh-log.txt
index d3674bbd2..f79685aa8 100644
--- a/docs/rh-log.txt
+++ b/docs/rh-log.txt
@@ -1,4 +1,10 @@
 December 6, 2007
+- New: On Windows, the game now checks the registry to see if you have Steam
+  installed. If so, it checks your SteamApps directory for any IWADs you may
+  have purchased through Steam and adds any it finds to the list of available
+  IWADs you can play. This means that if you bought your id games through
+  Steam, you can just extract ZDoom anywhere you like and run it without doing
+  any additional setup.
 - Fixed: The FLAC/makefile.mgw should use md5.o as an OBJ, not md5.c.
 - Fixed: The Linux makefile probably shouldn't be using the bundled zlib headers,
   either.
diff --git a/src/d_main.cpp b/src/d_main.cpp
index ec3369af4..734f4dd89 100644
--- a/src/d_main.cpp
+++ b/src/d_main.cpp
@@ -1629,6 +1629,26 @@ static EIWADType IdentifyVersion (const char *zdoom_wad)
 				}
 			}
 		}
+#ifdef _WIN32
+		FString steam_path = I_GetSteamPath();
+		if (steam_path.IsNotEmpty())
+		{
+			static const char *const steam_dirs[] =
+			{
+				"doom 2/base",
+				"final doom/base",
+				"heretic shadow of the serpent riders/base",
+				"hexen/base",
+				"hexen deathkings of the dark citadel/base",
+				"ultimate doom/base"
+			};
+			steam_path += "/SteamApps/common/";
+			for (i = 0; i < countof(steam_dirs); ++i)
+			{
+				CheckIWAD (steam_path + steam_dirs[i], wads);
+			}
+		}
+#endif
 	}
 
 	if (iwadparm != NULL && !wads[0].Path.IsEmpty())
@@ -1636,7 +1656,7 @@ static EIWADType IdentifyVersion (const char *zdoom_wad)
 		iwadparmfound = true;
 	}
 
-	for (i = numwads = 0; i < sizeof(IWADNames)/sizeof(char *); i++)
+	for (i = numwads = 0; i < countof(IWADNames); i++)
 	{
 		if (!wads[i].Path.IsEmpty())
 		{
diff --git a/src/win32/i_system.cpp b/src/win32/i_system.cpp
index 10850eb81..bdb4fd0c4 100644
--- a/src/win32/i_system.cpp
+++ b/src/win32/i_system.cpp
@@ -793,3 +793,45 @@ int I_FindClose (void *handle)
 {
 	return FindClose ((HANDLE)handle);
 }
+
+static bool QueryPathKey(HKEY key, const char *keypath, const char *valname, FString &value)
+{
+	HKEY steamkey;
+	DWORD pathtype;
+	DWORD pathlen;
+	LONG res;
+
+	if(ERROR_SUCCESS == RegOpenKeyEx(key, keypath, 0, KEY_QUERY_VALUE, &steamkey))
+	{
+		if (ERROR_SUCCESS == RegQueryValueEx(steamkey, valname, 0, &pathtype, NULL, &pathlen) &&
+			pathtype == REG_SZ && pathlen != 0)
+		{
+			// Don't include terminating null in count
+			char *chars = value.LockNewBuffer(pathlen - 1);
+			res = RegQueryValueEx(steamkey, valname, 0, NULL, (LPBYTE)chars, &pathlen);
+			value.UnlockBuffer();
+			if (res != ERROR_SUCCESS)
+			{
+				value = "";
+			}
+		}
+		RegCloseKey(steamkey);
+	}
+	return value.IsNotEmpty();
+}
+
+FString I_GetSteamPath()
+{
+	FString path;
+
+	if (QueryPathKey(HKEY_CURRENT_USER, "Software\\Valve\\Steam", "SteamPath", path))
+	{
+		return path;
+	}
+	if (QueryPathKey(HKEY_LOCAL_MACHINE, "Software\\Valve\\Steam", "InstallPath", path))
+	{
+		return path;
+	}
+	path = "";
+	return path;
+}
diff --git a/src/win32/i_system.h b/src/win32/i_system.h
index 1975d4c21..e17125d4c 100644
--- a/src/win32/i_system.h
+++ b/src/win32/i_system.h
@@ -207,6 +207,10 @@ extern const IWADInfo *DoomStartupInfo;
 // [RH] Used by the display code to set the normal window procedure
 void I_SetWndProc();
 
+// [RH] Checks the registry for Steam's install path, so we can scan its
+// directories for IWADs if the user purchased any through Steam.
+FString I_GetSteamPath();
+
 // Damn Microsoft for doing Get/SetWindowLongPtr half-assed. Instead of
 // giving them proper prototypes under Win32, they are just macros for
 // Get/SetWindowLong, meaning they take LONGs and not LONG_PTRs.
diff --git a/src/zstring.cpp b/src/zstring.cpp
index b74480938..db9d9d4dd 100644
--- a/src/zstring.cpp
+++ b/src/zstring.cpp
@@ -156,6 +156,15 @@ FString::~FString ()
 	Data()->Release();
 }
 
+char *FString::LockNewBuffer(size_t len)
+{
+	Data()->Release();
+	AllocBuffer(len);
+	assert(Data()->RefCount == 1);
+	Data()->RefCount = -1;
+	return Chars;
+}
+
 char *FString::LockBuffer()
 {
 	if (Data()->RefCount == 1)
diff --git a/src/zstring.h b/src/zstring.h
index 4d64261c6..ec21cfa53 100644
--- a/src/zstring.h
+++ b/src/zstring.h
@@ -130,6 +130,9 @@ public:
 
 	~FString ();
 
+	// Discard string's contents, create a new buffer, and lock it.
+	char *LockNewBuffer(size_t len);
+
 	char *LockBuffer();		// Obtain write access to the character buffer
 	void UnlockBuffer();	// Allow shared access to the character buffer
 
@@ -232,6 +235,7 @@ public:
 
 	size_t Len() const { return Data()->Len; }
 	bool IsEmpty() const { return Len() == 0; }
+	bool IsNotEmpty() const { return Len() != 0; }
 
 	void Truncate (long newlen);