My E51, Nokia supplied 512MB memory card started acting up like yours after I downloaded and installed some free wares from Nokia site. I stored them in my memory card and installed the wares.
I think the card doesn't have enough speed and when an application looks for the exe file and exceeds certain limits, the blame goes to the card.
So a permanent solution would be to save the files in C drive.
I havent tried this though, bacause I didn't want to clutter the drive and bog down the phone's speed.
What I tried to a certain success was this. But you'd probably need a desk and somewhere quiet to do this; not while you're driving or parachuting...
1. Remove the card from menu. Click power button ONCE, click the BOTTOM OPTION (remove memory card),
2. Take off the cover and physically remove the card.
3. Click OK on the menu. (phone lets out a small "peep"
4. Reinsert the card (another peep) and replace the back cover.
4. You're back in business.
5. If not, step 4 would be "Turn off the phone with memory card still outside the chamber."
6. Turn the phone back on.
7. Insert the card (another "peep".
Same problem comes back at most unexpected times. So annoying, but I read this problem happens in other Nokia models too, though I'm sure they wouldn't let this kind of thing out their lab doors if they knew it'd happen.
So, is it because their choice of memory card's vendor? Or the demo wares launching spam from memory card and taking up processor resources? Speaking for myself, I wouldn't want to store important data on it.
Formatting the card never made the problem go away, as I read in this and other forums.
(This a similar post I sent to Nokia forum 5 mins ago.)