I run it on Vista and it works fine.
Oh and here's a small guide on the classes in case you're wondering. (Good 15 mins spent on that. Was bored at the time so what the hey!)
If anyone feels something needs to be corrected in the class guide below, tell me and it will be fixed
Scout:
The scout is the fastest and weakest of all classes. His main purposes are to capture the intelligence (intelligence being equivalent to flag), and take control points. He is used as an offensive class, and a key one at that.
Soldier:
The soldier is slower than the usual 100% speed by a bit. He is armed with rockets and a high health bar. Another prime offensive character, known for being the average or middle of classes for way he works in a team atmosphere (you'll get what I mean when you play). Good for infiltrating bases and breaking through the front lines of enemies, he is strongest when armed with a Medic. Good for new players for being simple to use, the Soldier is surprisingly fun to play.
Pyro:
The Pyro is (not surprisingly) the pyromaniacs dream come true. Armed with a primary flamethrower, the Pyro causes Havoc offensively and defensively. He has a higher health bar than most and has a standard running speed. His flamethrower however, has a small range (almost like an extended melee weapon). His primary uses are for infiltrating enemy defences (offensive) and burning any intruders to crisps (defensive).
Demoman:
The Demoman is armed with fragmentation bombs and sticky bombs (sticky bombs stick to walls/celings/floors and are set off manually whereas, fragmentation bombs are lobbed and they explode after a few seconds). He is slightly slower than other classes with a speed rate of about 90%. Known as more of a defensive role by placing sticky bombs around certain choke points, they can also be used as an offensive class causing destruction to every base. If your aim is a little shaky and you love explosions the Demoman is for you.
Heavy:
The Heavy is the toughest of them all, and the slowest. With 3 times the health of most classes and half the speed of most classes he proves to be a great threat. Armed with a chain gun he is best paired with a Medic, healing him of any damages taken. Although made as a defensive class, he works spectacularly well as offensive, killing almost all classes with ease.
Engineer:
The Engineer is more of an indirect combat class. He is the defensive backbone of the team. While weak in direct combat, he can build sentries to guard himself/certain areas, let alone dispensers (which give out health and ammunition to teammates) and teleporters to different locations. If you prefer to strategise rather than be in the front lines than the Engineer is the class for you.
Medic:
The Medic is the healer in the team. Armed with a healing hose, he can stand behind a Heavy or Soldier (in most cases) and keep them alive and rack up points for assist kills. When you heal a Ubercharge meter fills. When it reaches 100% you can turn yourself and the healed person invincible for approximately 15-20 seconds. If you can time your Ubercharges right and keep yourself alive, your teammates will love you for it.
Sniper:
The Sniper is just that. Nearly useless in direct combat, the Sniper prefers to shoot off people's heads from 300ft away. If you have a steady hand and can quickly aim at people's heads than the Sniper is good for you. Oh and when you are scoped with your sniper rifle, a charge meter fills; increasing the power of your shot. When full it can kill some enemies with a body shot. Of course, it has drawbacks. When it charges, a dot (of the same colour as your team) appears, which is visible to the enemy and may tip them off.
Spy:
The Spy is the hardest but most rewarding class to play. He is the weakest in health but strongest in attacks in most cases. Armed with a knife, sapper, disguise kit and invisibility cloak, the Spy causes havoc in enemy lines. First off, the knife, if used behind an enemy player will kill them instantly IF it counts as a backstab (so get behind enemy, and stab their backs). The sapper can be used to disable and slowly destroy Engineer's buildings. It can be knocked off by 2 hits of a Engineer's wrench but if used well it's quite a sight. The disguise kit is used to disguise as a member of your team or the enemies'. Any shots fired will drop the disguise and bumping into enemies can tip them off as you cannot pass through enemies (through teammates you can). The cloak is used to temporarily turn yourself invisible and get behind enemy lines. If you prefer to deceive the enemy and act with stealth, the Spy is for you.