I do recall this being mentioned before, but I don't know what the solution was, if any. Out of curiosity, what version of the game do you have: box, jewel case, or download?
First off, if after you reinstalled you used the old save, that might mess things up. Try reinstalling and using the most stable patch configuration I know of: patch 2 and then temple.zip. (Details below)
Afterwards, start a new game and use the console to level up a ranger to level 5 real quick. (BTW, my test run just now had an archery-based ranger, so that's not the issue).
There is no THAC0 in 3.5. Rather, you have a + to your to-hit roll. This is increased by an amount given by the line of the table for the class level you just took. The first level of cleric gives you no plus to your to-hit, so taking your first level of cleric, no matter when you take it, will not boost your to-hit. But fighters and rangers both get +1 for every level, so any combination of ranger and fighter levels will give you a + equal to your total levels. Saving throws work in a similar way. Depending on your class and level, you might get a boost to one or more saves.
See here for more info on what each level specifically gets you:
www.d20srd.org/indexes/classes.htm
The only major downside to going fighter is you generally want heavier armor as a fighter, and if you wear medium or heavy armor you lose your ranger combat abilities (in your case, your archery feats).
Temple.zip details:
http://www.jadri.com/temple.zip
-or-
http://www.co8.org/files/patch/temple.zip
1. Unzip the patch. (Double click the folder you downloaded and choose the extract all files option in the corner of the window (XP))
2. Find the file in you GAME'S directory called "temple.dll" and remove it. (Put it somewhere you can find it again.)
3. Take the "temple.dll" file from the unzipped patch folder and put it where the original file was.
4. Inside the patch folder should be a folder called "data". Inside that folder should be three new folders. Take all of them and put them into the game's "data" folder.