The Rabanan stopped allowing a widow to swear because they were concerned for the punishment of a false Shevu'ah.
The reason why a widow is suspected to swear falsely is because of the justification that they put in a lot of hard work for the orphans; however a divorcee may swear before collecting her Kesuvah.
Shmuel holds that the prohibition for a widow to swear is only in Beis Din; outside of Beis Din she may take a Shevu'ah. (1)
Rav did not allow a widow to collect a Kesuvah at all even with a Neder. (2)
Rav admits that if the widow jumps up and makes a Shevu'ah that she may collect her Kesuvah.
Shmuel says that a widow who makes a claim on her Kesuvah in Beis Din may no longer receive Mezonos.
Once a widow is remarried she may no longer make a Neder in order to collect her Kesuvah because her husband may annul the Neder.
A husband may not annul the Nedarim that his wife made prior to the marriage.
It is a Machlokes if when a person goes to a Chacham to be released from his Neder he must detail the circumstances in which he made the Neder.
It is a Machlokes if a Neder that was made publicly may be annulled.
A Kohen who married a woman who is forbidden to Kohanim is Pasul for Avodah until he makes a Neder forbidding himself from benefiting from his wife.
Once the Kohen made the Neder he may engage in the Avodah even though he has not yet divorced her.
A BIT MORE
1. A Shevu'ah in Beis Din is more stringent because she must hold a Sefer Torah and she must swear with the name of Hashem.
2. Because in the time of Rav people were not careful with Nedarim and therefore he did not even allow he to make a Neder.
BROUGHT BY THE CLOUDS
A widow baked a Pikadon of a Dinar into bread and gave it to the poor and she swore that she did not benefit from the Dinar. She was punished for a Shevu'as Sheker because she benefited from the flour that she saved in the place where the Dinar was. Rashi says in Parshas Beha'aloschah that the Nesi'im were the last ones to donate to the Chanukas ha'Mishkan and therefore they brought Korbanos for the Chanukas Mizbe'ach. The Chasam Sofer asks that the Nesi'im donated very significant and valuable objects to the Mishkan, the Avnei Shoham and Avnei Milu'um. Rashi in Parshas Pekudei however, says that the Nesi'im did not bring the Avnei Shoham and Avnei Milu'um from their own money; the clouds brought them in their daily Manna. If so why is it regarded as a donation at all? The Chasam Sofer explains that although the Nesi'im did not spend any money on the Avnei Shoham and Avnei Milu'im, as each one of them found the Avanim of their in their Shevet in the Manna, however it is regarded as a donation because they were missing a little bit of Manna that morning; the space in the utensils where the Avanim was found.
When Beis Din or the inheritors make a widow swear before collecting her Kesuvah it must be outside of Beis Din because the Batei Dinim will not allow her to swear in Beis Din because of the concern that she will not be careful with her Shevu'ah. If the inheritors want her to make a Neder they may pick an item that they want her to prohibit and they make her take the Neder in Beis Din and she may collect her Kesuvah. If a widow dies before she made a Shevu'ah her inheritors may not collect her Kesuvah because she does not receive her Kesuvah without making a Shevu'ah. If she remarried prior to collecting her Kesuvah she may make a Shevu'ah and collect her Kesuvah, but she may not collect with a Neder because the husband may annul her Neder. (Rambam Hilchos Ishus 16:11, 12)