Try to patch game with the latest (and only one for now) patch. It should help.
Because Reimu is the goddamm armpit miko. You can't defeat her.
More likely answer, particularly if it keeps crashing after you've patched: Utsuho's up after Reimu, and the graphics on her cloak (the shimmering image of the universe on the inside of it, more specifically) can give a helluva lot of trouble for weaker graphics cards.
Solution: In your Hisoutensoku folder, there's a file called configex123.ini (this is the same file that you modify to tell Hisoutensoku where the SWR characters are located). Double-click on it, it should open up in notepad.
Scroll down to the very end, where it says "clientport=0". Press enter twice to open a space, then copy-paste:
And right under that...
Save the file afterwards. For some reason relating to coding (I honestly don't know enough to tell you), putting that text at the end of your config123.ini file makes it so Hisoutensoku limits the graphics of Okuu's cloak, allowing you to play against her without the game going haywire. You'll miss out on seeing the pretty stars in her cloak when she moves, but hey, better than not being able to complete Story Mode.