Confused about change?!

Hello,

I've been playing Pokemon HeartGold on my PC for about 12 hours or so without any lag/freeze whatsoever last week. Anyways, long story short my sister infected my PC with a virus when downloading something and I had to do a complete clean re-partition/re installation of Windows. Ever since then, I've been trying to play the same game with the same configs on NO$GBA but the game lags. In battle it plays fine but everything else, it drops down to 68% and fluctuates up and down a bit from that. I think I've got the same configs in NO$GBA/No Zoomer but I'm not 100% sure and the only difference is my PC use to be 1280x1024 resolution but now it's 1680x1050 (tried changing that with no luck with the lag).

PC Specs:
XP Pro Version SP3
Pentium 4 CPU 2.40GHz
768 MB of Ram

Like I said, the only difference in my PC between then (when it worked) and now is the resolution & this time, I've updated my system to SP3 w/automatic updates (use to have SP1 w/o any XP updates). Any help would be greatly appreciated!

Thanks.

...768 RAM I think your computer is running in Compatible Mode, Press F2 when your computer starts...I don't remember, but you changed your Ram also this happens to me to maybe its something running in the Background.

I feel that when there is no Lag its a good day to play emulators...even though its not the computer its what the computer is doing.