Messing around with
http://sheldonbrown.com/gears/
It looks like if you're going about 9 MPH in 39/25, then to get to 5.8 MPH at the same cadence, you'd need a 1:1 ratio between your smallest chainring and largest cog. To get this, you'd either want:
1) A triple crank with a 30T inner chainring and a cassette with a 30T largest cog (it's also possible to replace the 30T with a smaller chainring like a 28T and match to an 11-28 or something). This would require a new crank, new cassette, new rear shifter (if it's currently double-only), a triple front derailleur, a long/mid-cage road rear derailleur (or MTB RD), and likely a new/longer chain, or
2) A compact crank with a 34T inner chainring and a MTB cassette with a 34T largest cog. This would require a new crank, new cassette, a MTB RD* (both short/mid- or long-cage might work), and a new/longer chain.
Option 2 would probably be cheapest since it definitely does not require a new shifter (the most expensive part) or a new FD.
*If using a MTB RD with a road shifter, you cannot use the 10-speed Dyna-Sys; you must use a 8- or 9-speed version.
By the way, to see the numbers I was looking at, go to the Sheldon site above and input 26 x 1.25 for wheel size and MPH @ 80 RPM for gear units. These were inputted after trial-and-error so that a 39x25 gear outputs 9.1 MPH. You can add other chainrings and cogs to compare what your speed would be; this takes into account whatever your actual wheel size and cadence are.