diff --git a/HISTORY.rst b/HISTORY.rst
index bc535643b..c61181a29 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -28,6 +28,7 @@
 
 **Bug fixes**
 
+- 989_: [Windows] boot_time() may return a negative value.
 - 1007_: [Windows] boot_time() can have a 1 sec fluctuation between calls; the
   value of the first call is now cached so that boot_time() always returns the
   same value if fluctuation is <= 1 second.
diff --git a/psutil/_psutil_windows.c b/psutil/_psutil_windows.c
index bec2d3aa9..74e7cfef0 100644
--- a/psutil/_psutil_windows.c
+++ b/psutil/_psutil_windows.c
@@ -235,8 +235,12 @@ psutil_boot_time(PyObject *self, PyObject *args) {
     and 01-01-1601, from time_t the divide by 1e+7 to get to the same
     base granularity.
     */
-    ll = (((LONGLONG)(fileTime.dwHighDateTime)) << 32) \
-        + fileTime.dwLowDateTime;
+#if (_WIN32_WINNT >= 0x0600)  // Windows Vista
+    ll = (((ULONGLONG)
+#else
+    ll = (((LONGLONG)
+#endif
+        (fileTime.dwHighDateTime)) << 32) + fileTime.dwLowDateTime;
     pt = (time_t)((ll - 116444736000000000ull) / 10000000ull);
 
     // GetTickCount64() is Windows Vista+ only. Dinamically load
@@ -249,15 +253,15 @@ psutil_boot_time(PyObject *self, PyObject *args) {
     if (psutil_GetTickCount64 != NULL) {
         // Windows >= Vista
         uptime = psutil_GetTickCount64() / (ULONGLONG)1000.00f;
+        return Py_BuildValue("K", pt - uptime);
     }
     else {
         // Windows XP.
         // GetTickCount() time will wrap around to zero if the
         // system is run continuously for 49.7 days.
         uptime = GetTickCount() / (LONGLONG)1000.00f;
+        return Py_BuildValue("L", pt - uptime);
     }
-
-    return Py_BuildValue("d", (double)pt - (double)uptime);
 }
 
 
diff --git a/psutil/_pswindows.py b/psutil/_pswindows.py
index 52676183d..80225ee9e 100644
--- a/psutil/_pswindows.py
+++ b/psutil/_pswindows.py
@@ -410,7 +410,7 @@ def boot_time():
     # value which may have a 1 second fluctuation, see:
     # https://github.com/giampaolo/psutil/issues/1007
     global _last_btime
-    ret = cext.boot_time()
+    ret = float(cext.boot_time())
     if abs(ret - _last_btime) <= 1:
         return _last_btime
     else: