diff -ru xscreensaver-4.01-old/driver/prefs.c xscreensaver-4.01/driver/prefs.c
--- xscreensaver-4.01-old/driver/prefs.c	Thu Jan 17 05:29:04 2002
+++ xscreensaver-4.01/driver/prefs.c	Thu Mar 14 17:32:46 2002
@@ -1239,6 +1239,53 @@
   return h;
 }
 
+static int is_available(screenhack *hack)
+{
+	char *cmd, *path, *ppath, *tmp, *test;
+	struct stat sbuf;
+	static const char *def_path =
+#ifdef DEFAULT_PATH_PREFIX
+		DEFAULT_PATH_PREFIX;
+#else
+	"";
+#endif
+	
+	
+	
+	ppath = path = strdup(getenv("PATH"));
+	if (!path) return 0;
+	tmp = cmd = strdup(hack->command);
+	while (*tmp && !isspace(*tmp)) tmp++;
+	if (*tmp)
+		*tmp = '\0';
+	tmp = path;
+	while (path && *path) {
+		while (*tmp && *tmp != ':') tmp++;
+		if (*tmp == ':') {
+			*tmp = '\0';
+			tmp++;
+		}
+		test = alloca(strlen(path)+strlen(cmd)+2);
+		sprintf(test,"%s/%s",path,cmd);
+		if (!stat(test,&sbuf)) {
+			free(ppath);
+			free(cmd);
+			return 1;
+		}
+		path = tmp;
+	}
+	test = alloca(strlen(def_path)+strlen(cmd)+2);
+	sprintf(test,"%s/%s",def_path,cmd);
+	if (!stat(test,&sbuf)) {
+		free(cmd);
+		free(ppath);
+		return 1;
+	}
+	free(ppath);
+	free(cmd);
+	return 0;
+}
+
 
 static char *
 format_command (const char *cmd, Bool wrap_p)
@@ -1462,6 +1513,8 @@
   p->screenhacks_count = 0;
   while (start < size)
     {
+      screenhack *hack;
+
       /* skip forward over whitespace. */
       while (d[start] == ' ' || d[start] == '\t' || d[start] == '\n')
 	start++;
@@ -1474,7 +1527,9 @@
       /* null terminate. */
       d[end] = 0;
 
-      p->screenhacks[p->screenhacks_count++] = parse_screenhack (d + start);
+      hack = parse_screenhack(d + start);
+      if (is_available(hack))
+        p->screenhacks[p->screenhacks_count++] = hack;
       if (p->screenhacks_count >= i)
 	abort();
 
--- xscreensaver-4.01/driver/Makefile.in.foo	Thu Mar 14 17:47:04 2002
+++ xscreensaver-4.01/driver/Makefile.in	Thu Mar 14 17:47:21 2002
@@ -729,6 +729,11 @@
 	$(CC) -c $(INCLUDES) $(SUBP_DEFS) $(CFLAGS) $(X_CFLAGS) \
 	  $(srcdir)/subprocs.c
 
+# prefs takes an extra -D option.
+prefs.o: prefs.c
+	$(CC) -c $(INCLUDES) $(SUBP_DEFS) $(CFLAGS) $(X_CFLAGS) \
+	  $(srcdir)/prefs.c
+
 # demo-Gtk takes extra -D options, and an extra -I option.
 demo-Gtk.o: demo-Gtk.c
 	$(CC) -c $(INCLUDES) $(SUBP_DEFS) -I$(ICON_SRC) \