wine crashes my pc everytime i try to use it or open an exe file

Your whole computer? This should not be possible - if wine or the EXE file are broken, only wine should crash and nothing more. It could be possible that your computer is faulty... you can run memtest to check this. When booting Ubuntu there should be a boot menu with an entry called "Memtest" beneath the Ubuntu entries. Choose this entry and you should see a screen like this: http://www.memtest.org/pics/i875-big.gif

This program will run a complete suite of tests on your memory. You can see how far the suite has progressed with the "Pass" progress bar on top; as soon as this progress bar reaches 100%, the pass counter below increases by one and the test suite is run again (it will run indefinitely until you hit ESC). Wait until you have completed at least one pass. If the error counter is still 0, then you can rule out memory as the problem's source, but if there are errors, the memory or the mainboard could be defect.

Usually this means a mismatch between the kernel and the nvidia graphics driver...

  Ok, I reinstalled Ubuntu 8.04.1, ran the memtest, no errors. And got Wine from the Synaptic Package Manager. Still, Wine crashes my pc to the sign-in page.

It could be a bug in the X server or in your graphics driver.

What does
  lspci | grep -i vga
say?

Please try with Ubuntu 8.10, that may fix it, since it has newer X server and drivers.
Let us know if it does.

If you can still reproduce it, try one or more of the following:

1. run the wine app like this
  WINEDEBUG=+synchronous,+relay wine notepad > wine.log 2>&1
After the X server crashes, wine.log will show exactly what
Wine did to tickle the X server crash bug.

2. look at /var/log/X.0.log (or X.0.log.old) for X server errors

3. uninstall or stop gdm, log in via the console, give the command
  startx > x.log 2>&1
to start the X server, and reproduce the problem; when X
crashes, x.log will have a log describing why.
(This is a lot like #2 but has somewhat different info.)
Reinstall or restart gdm after you're done, so you can go back to
normal login.

Put the log(s) somewhere (probably compressed) and send us the
URL to them.