I'm looking for a PC real-time strategy game released between 1998 and 2002 that is similar to Cossacks: European Wars and Age of Empires 2

[Kantor]

That's oddly specific.

I'm going to be a little bit more obscure here and wholeheartedly recommend Warlords Battlecry II, one of my favourite RTS from back in the day. More fantasy involved there than in AoE2, and less structure (it's a non-linear campaign, like Dark Crusade), but it's great fun.

Here's a GR link. It's in the low 80s. http://www.gamerankings.com/pc/537053-warlords-battlecry-ii/index.html