With a computer like that, you'd be miles better off running an older version of FL Studio. I personally recommend FL 8. I've got the installer for the demo, and I'd be glad to upload it for you here, as it's quite hard to find anymore (if that's allowed - are we okay to do that, if it's free?).
Also, it would really help if you disabled some of Win7's visual effects. Press [Windows Key] + [Pause/Break] to bring up System. Click 'Advanced System Settings' (it's in the pane on the left). Under the 'Advanced' tab, click 'Settings...' under Performance. Click the radio button labeled 'Adjust for Best Performance' to disable everything, then enable what you need one-by-one. ('Smooth Edges of Screen Fonts', 'Enable Visual Styles', and 'Drop Shadows on Desktop Icons' are all you really need to have enabled, for the most part, but your mileage may vary.)
Look into some freeware VSTs by indie developers. Native Instruments is a solid company, but a lot of their plugs tend to use a lot of resources. Don'tCrac[k] is an AWESOME resource for free plug-ins. My favorites are Synth1, Minimouge (sic), LinPlug FreeAlpha3, and anything and everything by Xoxos. Avoid Image-Line's built-in synth Sytrus - it's a massive resource hog. :P
Let us know how it works out for you, and best of luck! If you have any questions regarding FL, feel free to ask them here - I know the software pretty well, and I'll help in whatever way I can.