Looks promising, but still...
The weapon upgrades make this look like it could be hours of collective fun, but other things defeat that. One: the slowdown. I have a fairly powerful pc (2.17 GHz, 512Mb RAM), but still the scrolling background (at low quality) starts to lag a little and the character gets a little jerky now and then. Maybe the background just have too much detail. The gameplay just isn't much fun. The characters should maybe be smaller, so Homestar isn't off screen most of the time while you're chasing him. Other than that, it's just: hold down arrow key. Shoot. Hold down arrow key. Reload. Shoot.
And frankly, while I tried not to make it influence the score, I think the way Homestar looks among the 'dark' backgrounds and with this character chasing him is rather silly. Like the game originally had a different target. I honestly can't see why you'd hate Homestarrunner enough to actually make a killing game about him, but I assume you've just spent too much time with avid HR fans.